THE GAP CONCEPT.
This is my own definition of what I believe Dan Harrington originally entitled 'The Gap Concept.'
The Gap Concept basically means that you should generally only Call a Big Raise, or Call and All In, with a Hand that is better than the Hand that is Raising or Going All In.
For example, if a Tight player goes All In and you hold JJ or less, or AQ or less, you should probably Fold, because if they have QQ against your JJ or less, or AK against your AQ or less, you are way behind. Calling here with QQ and AKo would depend on your stack size relative to the other player's stack and also relative to the Blinds (and Antes, if any.) However, you would probably call on balance with AKs, because the odds are very close and your opponent may only be pushing with AQo or AJo.
Some players, such a Joe Cada, have been known to gamble with any Pocket Pair in this situation, but this is very poor poker. Even IF your opponent doesn't hold a better Pocket Pair, you are gambling with only a very light advantage... The key to winning poker is NOT relying on Luck!