Hi there!
A mutation occurs when there is a slight change or, rather, mistake in passing down genes from an organism to its offspring or identical copy. The mutation may be very small and unnoticeable, and it may also be big enough to notice (such as better hearing). Mutations relate to evolution because when animals mutate, natural selection occurs, and as more and more mutations occur, different species may start to evolve, thus tying in with evolution.
Hope this helps!