A mutation is a change in the DNA. You could call it a slight mistake in the DNA code.
DNA contains genes. A gene is a piece of DNA that tells the body how something should be structured, such as a protein. Genes determine, for example, what you look like and how your body functions.
In humans and animals, a mutation can change a gene. That could affect a person’s appearance or body functions. This can happen naturally, but also be caused by something like sunlight or harmful particles in the environment.
Viruses such as HIV can also mutate and adapt themselves. HIV changes slightly as it reproduces itself, so the immune system no longer recognises the virus. This leads to many different strains of the HIV virus. That’s why it’s difficult to develop a vaccine or cure.