Occasional errors may occur during DNA replication.
In biology, a mutation is a change in the genetic material. This means changes to the DNA or to the chromosomes which carry the DNA. These changes are heritable (can be passed on to the next generation) unless they have lethal effects.
Mutations can happen for several reasons. It can happen because of errors when meiosis produces the gametes (eggs & sperms). Damage by radiation, or by certain chemicals may cause mutations. Mutations occur at random.
Also, by derivation, an individual carrying the mutation may be called a mutant or a mutation. So is the trait (character) most obviously affected by the mutation.