Originally Posted by In Tournament Poker for Advanced Players, Sklansky
There is a very important general principle understood by all good poker players. That is, you need a better hand to play against someone who has already opened the betting than you would need to open yourself. For instance, in jacks-or-better draw poker you might open with a pair of queens, but you would never call someone who has already opened when you hold those same two queens. In limit hold'em, you would most certainly raise in middle position with KQ especially if no one else was in yet but you would rarely play against an early position raiser with the same hand.
The difference between the hand you need to call an opener with, and that with which you would open yourself, I call the "Gap." How wide that Gap is depends on how tight your opponents play. If your opponents are quite loose, there may be no gap at all.