The rook  can be used (drop or move ) to threaten 2 pieces present on the same file.  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 rook on the king and bishop.

Here the bishop can not escape or the king will be captured so the bishop will be taken at next turn.

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

Other pins: