Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species evolve because of mutations accumulating over time.
The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variance. Some mutations are unfavourable or harmful and are quickly eliminated from the population by natural selection.
Others are beneficial and will spread through the population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce.
Some mutations do not do anything and can linger, unaffected by natural selection, in the genome. Some can have a dramatic effect on a gene and the resulting phenotype.