Checkmate state is a position of a chess game & in this position one player's King is Directly attacked by other payer's Chess pieces. First payer don't have any legal movement to protect his King. Normally in Chess games it has a practice to say that "Check" before Checkmate movement. But that is not necessary. Finally it can say that "a player who is checkmated loses the game".
Most of the time players resign before they get Checkmate. Sometimes game will Ends without a checkmate. That can be stalemate or Draw. stalemate is a State which player don't have any legal move and King is not under check state. There are lots of tricks behind to reach ultimate goal. In that Major Chess pieces will play major role in Later game. Queen & Rook are the Major Chess pieces.