The 'Kill Box' checkmate is a specific type of checkmate in chess. The term 'Kill Box' comes from military terminology where it denotes a zone in which a target is completely surrounded and vulnerable to attack. In this checkmate the opponent's king finds itself in a "box" formed by the opponent's queen and rook. If you draw the lines of attack by the attacking pieces a box will appear around the king in which it is confined.
The mated king is usually in the corner of the board or close to the edge but this checkmate is also possible in the center of the board if the king is either blocked by its own pieces or restricted by opponent's pieces.