Checkmate is a chess move that makes it impossible for your opponent to win. A checkmate can also be any kind of clear victory.
In chess, a checkmate is a move that leaves your opponent with no more options: there is no move that won't result in you getting their king and therefore ending the game. Since a checkmate is so final, the word is used for other kinds of final victories. In basketball, a three-point shot that seals a win could be called a checkmate. In politics, a speech that wins votes and wraps up an election could be called a checkmate.