The mutation rate of a biological species is the probability of error in gene replication. A gene replicates with a low error rate, on the order of 1 in 100 million, which is sufficient to provide the essential material for evolution.
Enzymes replicate DNA which correct errors spontaneously. Since the enzymes themselves are produced by genes, the mutation rate is under genetic control.
Since the majority of mutations are harmful, too high a mutation rate will produce less offspring, yet too low a mutation rate will inhibit evolutionary adaptation.