As this article says, "[Rehearsal cubes] are used as stand-in props to help block out or act out a scene. There are endless combinations and with a little imagination, they go a long way. Stack up two cubes for a table. Use one as a chair. Put three of them together on the floor to make a bed. Turn one up so the open end is facing the audience to act as a TV set or cupboard. Use them as podiums or soapboxes to stand on. Stack three, two, then one to make a staircase or represent mountains. The possibilities are limitless." Built with an open end they double as storage containers between shows.
Click here for directions and plans to make rehearsal cubes.