Usually there are three ways in which a mutation can affect a trait of an animal; mutation effects can be
A) neutral, that is, the mutation doesn't cause any change;
B) negative, where the change caused by the mutation can be detrimental for the animal;
C) positive, where the change caused by the mutation can give an advantage to the animal.
The last two effects play a major role in evolution; when the mutation causes a positive effect, there is an increased probability that the animal (or the plant) will reproduce and pass it to their offspring (their fitness is increased). When the mutation has a negative effect, the probability of survival and reproduction of the animal is reduced (their fitness is reduced), and consequently it will be likely that this mutation will be negatively selected, thus disappearing from the population.