A friend of a friend; one that perhaps you've never met or you aren't well acquainted enough with to call them a proper friend, but your friend is close with.
The 'once' part can be changed depending upon how close this friend is; Your friend's friend's friend's friend would be your friend-thrice-removed.
I was talking with Jen the other day. Turns out my friend-once-removed Darren is in a band that's playing at the club on Friday. Wanna come see them with us?