A Mutual Friend is a Friend who is a common friend between you and your other friend. So if you are A and your friend if B if there is a 3rd friend C who is a friend of A & B will be called as Mutual Friend.
Mutual friend means that say for example "johnny" is the person you want to add. And Lola the friend you do have is friends with johnny, it means one of your friends is already a friend of that person.
A friend that knows your other friend quite well
It's someone you and another friend have in common and he's not on your friends list