Myths are the stories a culture tells to explain big things like the origins of the universe, why the sky is blue, and where socks go when we lose them in the dryer. A myth more than likely involves gods or supernatural elements (we're blaming you, Loki, for the socks thing).
These days, the word "myth" has become synonymous with "lie"—it's bogus (i.e. "Midlife crisis is a myth!"). But way back when, it had a much deeper, much cooler meaning. A myth was a sacred story, an important part of religion, and helped humans understand how they fit into the world. It's pretty cool stuff.