3 Betting is the situation in poker where the inital raiser is raised. If you are confused why a 3 bet is called a 3 bet, think of the big blind as the first bet, the initial raiser as the second bet, which makes the reraise of the inital raiser the 3rd bet: hence the term 3 bet.
There are 3 main reasons to 3 bet:
1) Enlarge the pot to get value from a premium hand preflop. Hands like the following would be 3 bet for value:
kk QQ AK
2) Have the initiative in the hand with a speculative hand with the intention of either flopping the best hand, bluffing or semi-bluffing post-flop. Suited connectors, suited aces and suited one gappers are the best hands to 3 bet in this instance. However we should only really be 3 betting in this situation if we have position. Position alone can be the difference between making this 3 bet profitable or a losing play.
3) To bluff a player that we believe to be raising too many speculative hands that we think will fold a high percentage of the time to a 3 bet. Here good candidate hands to 3 bet are: suited connectors, suited aces, suited one-gappers and suited hands with one big card. Again here we should only be looking to 3 bet if we have the advantage of position! If called we should be looking to semi-bluff, pot control with top pair or bluff sometimes on boards that we do not think connect with our opponents 3 bet-call range.