Mutation is the genetic mechanism that introduces new alleles by changing DNA sequences.
Mutations can result from copying errors, chemical changes, radiation, or other forms of DNA damage. They may involve a single nucleotide, larger sections of DNA, or changes in chromosome structure. Most mutations are neutral or harmful, while a smaller number can be beneficial in a particular environment.
Mutation is the ultimate source of new genetic variation, but mutation alone does not necessarily make a population evolve in a directed way. Natural selection, genetic drift, migration, and other processes determine how variants change in frequency.
A common mix-up is treating every mutation as an adaptation. An adaptation is a heritable trait that improves reproductive success in a particular setting; a mutation is simply a DNA change. A mutation can provide the raw material for adaptation, but its effects depend on context.