A mutation is a change in the genetic material (the genome) of a cell of a living organism or of a virus that is more or less permanent and that can be transmitted to the cell’s descendants or the viral progeny.
While mutations are a major contributor to evolution they are not always meaningful and in the case of pathogens not always a scary thing. Mutations happen all the time, both in microbes and larger organisms such as humans. Additionally, most mutations don’t actually result in any noticeable change. This is because mutation is a random process. Randomly changing nucleotides in the genome of any organism or virus will much more often be deleterious than beneficial. However, because of the constancy of mutation in all organisms some mutations will arise which may bolster the fitness of an organism or virus.