3-Bet
The first re-raise before the flop. The big blind counts as the first bet and the open raise as the second.
If a player opens and you raise again, you have 3-bet. A raise over a 3-bet is a 4-bet. After the flop, "3-bet" simply means any re-raise of a raise.
Players 3-bet strong hands for value and some weaker hands as bluffs, often suited hands such as A5 that play well when called.
Example
The cutoff opens to $6, you 3-bet to $20 from the button.