'Corner mate' is a type of checkmate in chess in which the opponent's king is driven into a corner of the board. As the name suggests the opponent's king is in the corner of the chessboard or is forced to retreat there. In the corner of the chessboard the King has the fewest moves so this is the place where it is easiest to checkmate - by restricting or blocking the king (for example with your own pieces).
Usually heavy pieces (queen and/or rook) are used to checkmate a corner mate but variations with other pieces are possible. Also many checkmates can only be done in a corner (for example 'Checkmate with two bishops' 'Checkmate with a knight and bishop' or 'Morphy checkmate').