Ape index is a body-proportion metric: the difference between arm span (fingertip to fingertip with arms extended horizontally) and standing height. A climber with a positive ape index has arms longer than their height; a negative ape index means arms shorter than height. The metric is named for the great apes, whose arm span typically exceeds their height by a noticeable margin.
A positive ape index can be an advantage in climbing because it lets a climber reach holds others have to dyno or set up moves for. On routes with long reaches between holds, an extra few centimetres makes the difference between flowing through a sequence and having to find a more powerful alternative beta. The most famous example is Adam Ondra, who has an ape index of around +6 cm.
That said, ape index is often overrated. Plenty of world-class climbers have neutral or even negative ape indexes — they work the routes differently, using better footwork, more compression, or more dynamic movement. Technique, finger strength, and route reading determine performance far more than reach. Treat ape index as a curiosity, not a ceiling: how you climb matters more than the proportions you climb with.