That's the half hitch. Bring the end around the standing end of rope and back through the loop you just made, creating another half hitch to complete the knot.

there must be a tail or bight of rope to pass through a loop. Once this is completed, a knot will form. A hitch is created when you attach one end of the rope to an object or another rope.

The Mule Knot (a.k.a releasable knot) is ideal to lock off the rope on the belay plate. There are many occasions in climbing when, belaying from the harness using a device which does not self-arrest ...