What defines the Main Line?

#1
The main line is what is played most.
That changes over time. Sometimes another line takes over as the main line.

I think opening principles are the most important thing, but I also see the benefit of studying a certain opening.

Now in the opening books, there are full games, with more games as sidelines. If you go through them, you can get an idea of what the ideas are in that opening. The pawn structure. The strengths and weaknesses and how the masters played those positions.

So then when you play a similiar position, maybe you remember something and get a hint of what to do when you feel lost.

I agree that openings are important even at the beginner level. I think it would be beneficial to study and get good at one or two openings for each side instead of studying all of them for now though.

I always thought that the "main line" was the one that top players consider to be the most fundamental test of the soundness of the opening, and for which there is a lot of theory, and for which the outcome given best play is still unclear.  I don't think it's necessarily the one that is most popular, since a lot of players avoid the main line because of the amount of memorization required.

But I could be wrong.

Hi, I'm a beginner and have been playing in chess.com for one year. Now I can maintain my ELO above 1000.What to do for improving my play to the next level ? What are the concepts to focus ?