Main line

I am studying an openings book by Yasser Seirawan what is a Main Line in chess openings?? Can anyone enlighten or broaden my opening horizons??

The line that is played most often at the highest levels (everything else is considered an offshoot).

It's the most played and explored line, the one which is currently reputed "best" by the majority of the top players. Of course a main line can fade in importance with the time and a sideline can rise to the status of new main line. For example in the early days of chess 1.e4 e5 2.Nf3 Nc6 3.Bc4 was definitely the "main line" of the open games, but the variation slowly faded in importance and now 3.Bb5 is much more popular.

good stuff.