You must find the Queen in order to achieve CHECKMATE!
The Queen's Gambit is a move designed to secure control of the center of the board. It's one of the most common chess openings and involves white sacrificing (that's the "Gambit" part) a queen-side pawn (the "Queen" part).
In Algebraic notation, the move is:
White moves its pawn to D4. Black moves its pawn opposite to D5.