This literally means the third bet. The situation when you bet, after someone has already bet (not limp!) After the blinds. For example:
Player 1 - SB
Player 2 - BB
Player 3 - fold
Player 4 - Raise 2 BB
Player 5 - fold
Player 6 - Raise 10 BB - 3Bet raise
...
And this is called squeeze:
Player 1 - SB
Player 2 - BB
Player 3 - fold
Player 4 - Raise 2 BB
Player 5 - Call 2 BB
Player 6 - Raise 10 BB - 3Bet raise
...
3-bet is the third raise in the round of trade after the blinds and raise.
Actually is the 2nd raise bcs the blinds count as the 1bet the raise as the 2 bet and the reraise as the 3bet