A mutation is any permanent change in the DNA sequence.
A mutation may affect only a small part of the molecule, and thus be confined to one gene, or be much larger, perhaps affecting an entire chromosome.
Most mutations are harmful, or have no effect at all. A few are beneficial, for example, even a slight change in colour of a prey species may make it harder to be seen by a predator.
The arising of various mutations within a population of a species provides a mechanism for evolution by natural selection; the different mutations will have different survival rates.