A T-slot nut is used with a threaded clamp to position and secure pieces being worked on in a woodshop. The T-slot nut slides along a T-slot track, which is set in workbench or table for a router, drill press, or bandsaw. T-slot nuts are also used with extruded aluminum framing, such as 80/20, to build a variety of industrial structures and machines.
It is very similar to the "channel-and-groove design" used in some toys.