Mythology is the study of the stories which explain the culture, beliefs and other events of a particular group of people. These folk tales often explain things like how the world came into being, or what happens to the Sun after it sets, etc. They’re very valuable insights into different cultures, and they’re also relevant when it comes to understanding the human psyche.
Myths should not be confused with legends. Myths can be (and usually are) totally fabricated, while legends usually have some root in factual people or events.