What does the mutation rate do?

The mutation rate in this simulation is the probability per offspring for mutations to occur at all.

For example, if you set the mutation rate to 25%, then for each creature created through recombination of its two parents there is a 25% chance that its chromosome will additionally also be randomly mutated.

Note that this is just a probability and not a guarantee. The might be a generation with 0 or 100% mutated creatures. The larger your population size is, the closer the actual number of mutated creatures tends to be to what is predicted by the mutation rate.