A strike plate is a metal plate affixed to a door jamb with a hole or holes for the bolt of the door. When the door is closed, the bolt extends into the hole in the strike plate and holds the door closed. The strike plate protects the jamb against friction from the bolt and increases security in the case of a jamb made of a softer material (such as wood) than the strike plate.
A strike filler plate is used for covering up holes and cut-outs in the frame in place of door handles when it is not used or needed. Filler plates can improve the appearance of the frames when used. Most door and frame manufacturers create cut-outs in these components at the factory.