An internal wrenching nut, also known as an Allenut or Allen nut, is a cylindrical nut that is internally threaded

Screw thread

A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread...

 on one side and has an Allen socket on the other side; the outside of the nut is smooth or has knurling

Knurling

Knurling is a manufacturing process, typically conducted on a lathe, whereby a visually attractive diamond-shaped pattern is cut or rolled into metal.- Uses :...

 on it. The Allen socket may be 6 point or 12 point (also known as a double hex socket). They are used where hex or square nuts won't fit.