It's the "fifth" cusp, and is apparently present in about 60 % of upper first permanent molars. The "typical" upper first molar has four cusps: mesio-buccal, mesio-palatal, disto-buccal and disto-palatal. This extra cusp is usually on the palatal side on the mesio-palatal cusp - like an extra projection hanging off the tooth that can be felt by the tongue.