The overhand is the simplest knot possible.
In canyoning, it is most useful as a stopper knot, tied in the end of the rope to prevent you from abseiling off the end.
It can also be used to tie loose ends off around the main strand of rope, or back up knots such as the rethreaded figure-of-eight or figure-of-eight on a bight.
|Pass the end of the rope over itself...|
|... and then back through the loop formed.|
|Pull tight and set.|