What is a mutual friend on facebook?

A mutual friend is the person who is also friends with one of your friends. Example: If you have a friend named Suzy who is friends with a girl named Rose, Rose would be your mutual friend because you and her are both friends with Suzy. Basically mutual friends are people who are friends with the same person as you are.

Sorry if this answer was confusing, but this is the idea of what mutual friends are.