"mutual friend" is a pseudonym for the internet, or an internet dating site, when used to cover "how do you know...?" "oh, we met through a mutual friend"
"how do you know so and so?" "Oh, we met through a mutual friend" (translation: we met through an internet dating site looking for freaky people who like this and that...)
friends of the same friends
tom meet jerry through bob. bob is friends with both tom and jerry. so now tom and jerry are friends.
soooo tom meet jerry through a mutual friends/friend.
These are friends on Facebook that you and another person have in common. That is, both of you know those people.
Robbie and I have 180 mutual friends on Facebook. How do I still not know who he is?!