锐 is a bound form (b.f.) that appears as a morpheme in compound words. It carries the core meaning of 'sharp' — both physically (as in 锐利 'sharp blade') and mentally (as in 敏锐 'keen perception'). It also extends to 'rapid' in words like 锐减 'sharp decrease'. You cannot use 锐 alone as a word; it must combine with another character.