An undercling is a hold and grip type where the palm faces up — the climber grips the bottom edge of the hold and pulls outward, away from the wall. The grip works because of opposing tension between the upper body (pulling out) and the feet (pushing back into the wall). When the body is properly aligned, underclings can feel surprisingly secure.
Underclings are most useful below the body — on a hold beside or just below the chest, the climber can press their feet hard against the wall and use the undercling to lever themselves up to the next hold. Above the body, underclings are far weaker, because the geometry no longer creates useful opposing tension.
Classic undercling moves include levering off a flake while reaching for a high hold, holding tension on a roof while moving feet, and matching feet on a smear under the hand. They're a key vocabulary move once climbers move past V2 or 6a.