The bishop  can be used (drop or move ) to threaten 2 pieces present on the same diagonal.  If one of the piece is on the same file as the king, then it could not escape and it will be captured at next turn.

Some example :

Pin from the bishop on the king and rook.

Here the rook can not escape or the king will be captured so the rook will be taken at next turn. This pin is very useful.

Pin from the bishop on the rook and a pawn allowing the promotion of the bishop.