Matthews Apple Boxes are one of the most ubiquitous and useful pieces of equipment on a film set. They are used for anything that needs to be propped up or supported temporarily. They can be used to prop up furniture and light stands, for leveling camera dolly track, or to provide temporary seats, workbenches, or stepladders.
You can nail wall plates to them to hold low angle lighting fixtures and a multitude of other purposes. Matthews Apple Boxes are constructed of industrial grade plywood.
Often the need arises to make an actor appear taller, either because of their height, or to fit with the composition of a particular shot. In this use apple boxes are jokingly referred to as "man makers".