不会 (bú huì) is the negation of 会 (huì), which can mean will (predictive future), know how to, or can. Accordingly, 不会 means won't (prediction), cannot / don't know how to (skill), or would not (expressing improbability). Context clarifies which sense is intended: 不会游泳 (cannot swim - skill), 他不会来 (he won't come - prediction). It is one of the most frequent negative expressions in everyday Mandarin.