It means an opening where a player makes some sacrifice (mostly a pawn), for getting an advantage (better position) 
What is a gambit?
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Jan 26, 2021
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On a simple level a Gambit is a pawn sacrifice. You offer your opponent a material advantage for a time advantage that gives you a lead in development. The idea is your opponent makes one or more extra moves capturing a pawn and maybe retreating an attacked piece
sorry. but you use the time to develop more pieces and castle your king to safety so you have a lead in development. A lead in development is normally a temporary advantage, your opponent can catch up in which case you have nothing to show for the pawn, so often the idea is to convert the development advantage to something more enduring such as a bigger material advantage via a combination or a positional advantage. So a Gambit is a bigger idea than a simple sacrifice.
BTW. I think a gambit is the offer of a pawn sacrifice rather than the sacrifice itself. It can be either accepted or declined.
It doesn't have to be just a pawn sacrifice. Some gambits give up more than one pawn, and there's even stuff like the Halloween Gambit that sacs a knight on the 4th move of the game.
Look up the Halosar Trap which is a game. It's full of gambits. White wins in six moves after being down two pawns and a queen.
People keep talking about gambits. What exactly is a gambit