A climbing technique hooking your heel onto a hold to pull your hips close to the wall.
A heel hook turns your leg from a pure support into a pulling limb, taking load off the arms that otherwise fail first. On overhangs it often replaces an entire pull-up. Technically you pull actively with the back of the leg rather than just hanging the heel on: hamstrings and glutes do the work while you rotate your hips towards the wall. Two things to watch: place the heel precisely on the edge rather than half off it, and load it gradually — a heel hook loaded abruptly with a deeply bent knee is one of the most common causes of meniscus and hamstring injuries in bouldering.