Fork
A fork is one move that attacks two or more enemy pieces at the same time, so only one of them can be saved.
Knights are the usual forking piece because they jump and change color each move. Pawns, bishops, rooks, queens, and even the king can fork when two targets sit on squares that one piece attacks.
Black plays Nc2+ and hits the king and the queen together. White must step out of check and the queen falls. A fork that includes the king is the most forcing kind, because the reply is not optional.
If you leave two loose pieces on knight hops from the same square, the coach will treat that as a fork waiting to happen.