Mutations are changes in the nucleotide sequence of DNA and cause new genes and alleles to arise making them sources of genetic variation and therefore one of the things that musn't happen to maintain genetic equilibrium.
Molecular clocks use mutations that have no effect on phenotype because when comparing their DNA sequences to other species, they can reveal how many mutations have occurred independently from one another.
During meiosis, when eggs and sperm cells are formed a mistake could occur when DNA is being copied resulting in a mutation
An example of a mutation would be if a codon was misplaced resulting in an incorrect DNA sequence. This can be beneficial or negative, such as sickle cell disease.