Clearance

Occasionally a position arises where a combination would be possible – were it not for one of your own pieces standing in the way. If you move this piece way with a forcing move, you can proceed with the combination. This tactic is called a clearance: the forcing move clears a square or lines for other pieces.

Since the initial clearance move is not related to the combination, it can easily be missed by the defending side. In order to be successful, the clearance move obviously has to be a forcing move. Otherwise, defense against the combination is still possible.

What do you have to do?
Play a forcing move to clear the way for a combination.