A toe hook is the inverse of a heel hook: instead of pressing the heel down on a hold, the climber pulls a hold with the top of the foot — toes pointed and the foot rotated inward. The shoe rubber on the upper surface of the toe box grips the hold while the leg pulls inward toward the body.
Toe hooks shine on roofs and steep overhangs, where the climber needs to pull their body up against gravity and the foot can't simply press on a hold. By hooking with the toe, the climber turns the leg into an extra pulling limb — the same role a heel hook plays but with the foot in a different orientation. Many roof problems require both feet to toe hook simultaneously, with the climber compressed in tight against the rock.
The shoe matters enormously. Aggressive bouldering shoes with full rubber wraps on the toe box grip far better than gym rentals or flat shoes. Without that rubber, the toe slides off. Toe hooks fatigue the calves and the front of the shin quickly — strong shins are an underappreciated requirement of advanced bouldering.