Hanging piece
A hanging piece is an unprotected unit that can be taken for free, or for more than it is worth.
“Loose pieces drop off” is the short version. If two of your pieces are unprotected, a fork will take one of them. If one piece is unprotected and attacked, it is already hanging.
The knight on e4 has no pawn in front of it and no piece defending it. Both white knights attack the square. Nxe4 wins a piece.
The coach uses this idea constantly: a move that leaves a piece hanging, a capture of a hanging piece, or a threat you ignored. Before you look for a brilliant tactic, check whether something is simply free.