Strike plate

The strike plate, also called the keep, is the metal part fixed to the door frame opposite the lock, with the openings that the bolt and the latch go into when the door is closed. Without a strike plate, the bolt would go straight into the timber of the frame.

It matters more than it seems: when someone kicks or levers the door, it is the strike plate that takes the strain. A thin one held with short screws tears a chunk out of the frame and the door opens, however good the lock is.

If you are going to reinforce a door, start here. A steel strike plate with long screws that reach into the wall greatly improves its resistance without changing the lock. More under lock installation and door reinforcement.