The Gap Concept

A key to tournament poker
Tournament poker requires special strategies and tactics. One thing you have to be familiar with is the gap concept. This concept was introduced by the famous poker theorist David Sklansky and it's actually rather simple - in a poker tournament, you will generally need a better hand to play against someone who has opened the betting than what you need to open the betting with yourself. The difference between the two mentioned hands is called the gap.

Using the gap
The width of the gap gets bigger if you're facing a tight opponent and smaller if you're facing a loose opponent. As a result of this you should make raises with many hands you would never call a raise with. If you have late position and a decent stack in a Hold'em game and no one has opened the action, you might well raise with hands as weak as A-x, K-9s and 2-2. However, you should always be aware of extremely aggressive players. If a player that often plays back is sitting in the blinds you have to be careful and more selective.

Source: WSOP