why kingside majority so important?

i heard thats important but dont know why. would you explain it ?

So important? Compared to what?  A queenside majority?

There are whole books on chess strategy explaining the concepts  of pawn structures and queen or kingside majorities and why when you get to the endgame, a queenside majority may be more valuable than a kingside majority because a passed pawn would be farther away from the defending king and therefore more likely to be queenable.

See Queenside Pawn Majority 

A Google search for "kingside majority" will also take you to Jeremy Silman's "Complete Chess Strategy" and a whole chapter explaining why the queenside majority is usually more important than a kingside majority.You can get a preview of one page of the chapter. 

Ludek Pachman's "Modern Chess Strategy" also has a chapter on it.

There are probably dozens of other books that have covered the subject.