Mutants are beings that differ genetically from their species norm, and may display phenotype differences as well (changes in appearance, added abilities or resistances, or perhaps vulnerabilities as well.) Because mutants are 'outside the norm' they are often looked upon with suspicion, fear, and prejudice in whatever society they come from.
Keep in mind that mutants are beings whose genetics dictate what sorts of powers they have, as opposed to those beings of the Science origin whose powers are acquired from an outside stimulus (e.g. chemical spills, exposure to radiation, scientific experiments, etc.).