Definition
A 3-bet is the action of re-raising an initial raise. The betting sequence is: blind (1st bet) → raise (2nd bet) → 3-bet (3rd bet). If there is a subsequent raise, that becomes a 4-bet, and so on.
3-betting preflop is a core part of modern poker strategy. A 3-bet range typically consists of a "linear" range of the best hands (AA, KK, QQ, AKs) and some "bluffing" 3-bets with hands that benefit from fold equity but have some equity when called (suited connectors, suited aces).
The size of a 3-bet is important: too small and you give the original raiser good odds to call and realize their equity in position; too large and you charge yourself too much with your bluffing hands. Standard 3-bet sizes range from 3x to 4x the original raise out of position, and 2.5x to 3x in position.
Example
The player under the gun opens to 2.5BB. You are in the Cutoff with K♥Q♥ and 3-bet to 7.5BB. The Big Blind "4-bets" to 22BB. Now you face a decision about whether KQs is strong enough to call a 4-bet.