What exactly is "theory" when discussing openings?

  Theory is the moves you find in databases and books that have already been tried and the statistics tell how well those moves have worked. The moves that work best are found in books along with explanations of why they work well.

I think it helps to know the book moves. At least the first few. Your opponents will try to take you out of book, and if you know the "why" of the book moves then you might be able to figure out the weakness in your opponent's non-book moves. This doesn't always require memorization. 

In the case of the KIA, you are striving to set-up a formation. All you have to memorize is the formation, and a few move order tweaks, in order to reach the middle game.

Openings don't win games.. The purpose of the opening is to reach the middle game with equality or better. I think the beginner should stick with just one or just a few openings. Then you can focus your attention more on tactics and endgame, because that's where games are won and lost.

Here is the KIA formation that you should be able to memorize without much effort.

White"s move order is typically 1.Nf3 2.g3 3.Bg2 4.0-0 5.d3 6.Nbd2. These moves all support the pawn push e2-e4. You may have to make move order adjustments due to your opponent's moves.

Welcome to the club. Don't be afraid to ask questions.

As explained, theory is basically game and time tested opening lines or a number of moves. It really should be called current theory because it can change over time. Players can come up with new moves, called novelties, that may be an improvement. If it is better, then that becomes the latest theory.

Each opening usually has different variations and these variations can have different lines. Like queens gambit declined can have a Tarrasch variation or Ragozin variation but each variation also has different move branches or lines.

Opening principles guide all opening theory but there are going to be trade offs with principles. So different openings, variations and different lines are basically different principle trade offs.

If you are very comfortable with opening principles, you will understand what each opening move means and then that will help when it's necessary to memorize or if you get to some unfamiliar opening.

Discovering Chess Openings: Building Opening Skills from Basic Principles

By John Emms.  

Many chess teachers advise against spending excessive time on the study of openings variations (i.e., "theory") for the beginner, but they do agree it is important to acquire a solid understanding of opening PRINCIPLES right from the start.  This book explains the basics of opening FUNDAMENTALS better than any other book.  If you are uncertain about what you should be thinking and doing during the opening, this book does an exceptional job of addressing these issues.  For additional instruction in chess opening fundamentals, see also "Chess Openings for Juniors" and "Winning In The Opening", both by J.N. (John) Walker.  For instructive (and entertaining) examples of how to exploit mistakes and inaccuracies in the opening, see "Chess Openings: Traps and Zaps" (2 volumes) by Bruce Pandolfini.

"... Overall, I would advise most players to stick to a fairly limited range of openings, and not to worry about learning too much by heart. ..." - FM Steve Giddins (2008)
"... Once you identify an opening you really like and wish to learn in more depth, then should you pick up a book on a particular opening or variation. Start with ones that explain the opening variations and are not just meant for advanced players. ..." - Dan Heisman (2001)
https://web.archive.org/web/20140626180930/http://www.chesscafe.com/text/heisman06.pdf
"... To begin with, only study the main lines ... you can easily fill in the unusual lines later. ..." - GM John Nunn (2006)
"... I feel that the main reasons to buy an opening book are to give a good overview of the opening, and to explain general plans and ideas. ..." - GM John Nunn (2006)
"... If the book contains illustrative games, it is worth playing these over first ..." - GM John Nunn (2006)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)
"... For inexperienced players, I think the model that bases opening discussions on more or less complete games that are fully annotated, though with a main focus on the opening and early middlegame, is the ideal. ..." - FM Carsten Hansen (2010)
"... Everyman Chess has started a new series aimed at those who want to understand the basics of an opening, i.e., the not-yet-so-strong players. ... I imagine [there] will be a long series based on the premise of bringing the basic ideas of an opening to the reader through plenty of introductory text, game annotations, hints, plans and much more. ..." - FM Carsten Hansen (2002)
https://web.archive.org/web/20140627055734/http://www.chesscafe.com/text/hansen38.pdf
"The way I suggest you study this book is to play through the main games once, relatively quickly, and then start playing the variation in actual games. Playing an opening in real games is of vital importance - without this kind of live practice it is impossible to get a 'feel' for the kind of game it leads to. There is time enough later for involvement with the details, after playing your games it is good to look up the line." - GM Nigel Davies (2005)

To learn openings playing three-days a move online is very good. Maybe ten games simoultanously. The daliy onlinechess has the explorer-tool, an openingdatabase, so you can check which moves are popular among masters. I first find my move, then check with explorer, I explorer agree, I move it. If explorer has other opinions I think about if I like them better. Sometimes I dont understand the explorermoves and clicks  trough the whole mastergame to see where it leads.

Theory is just theory; it is data, it is the full body, good, bad, indifferent, of what has already been played and tested and analyzed. Current theory is what is currently popular, or thought to be best. 

Theory has never been the best word. "Generally agreed tried and tested practice" would be a better, if less pithy, term.

I'm in the same boat.  

My "study plan" has been to play using 'opening principles' and read enough stuff in the forums to not fall immediate victim to some "Banana in the Tailpipe" gambit (1) like the fishing pole, poisoned pawn, fried green livers, and such.   Danny Rensch has good 'opening principles' video on YouTube.

(1)  Banana in the Tailpipe reference:   https://www.youtube.com/watch?time_continue=8&v=HktV2yGtLv8

Noted. Typical. 

Best to completely ignore those who have nothing positive to contribute.  They are looking to evoke responses in order to continue defecating their negativity.  Without the attention of others to feed on, like weeds deprived of water they will eventually dry up and blow away.

"... In the middlegame and especially the endgame you can get a long way through relying on general principles and the calculation of variations; in the opening you can go very wrong very quickly if you don't know what ideas have worked and what haven't in the past. It has taken hundreds of years of trial and error by great minds like Alekhine and, in our day, Kasparov to reach our current knowledge of the openings. ..." - GM Neil McDonald (2001)

My impression is that "theory" is a word that is sometimes used to refer to that accumulated knowledge about openings. If I remember correctly, there is a story that Alekhine was sitting at a board and making some comments on some opening while several other players stood around. The story goes that someone interrupted, saying, "But according to theory, ...". Alekhine himself interrupted, banging down a piece and proclaiming, "I am theory!"