The boundary of a friendship. It refers to the type of relationship you share with a person and the subject matter/actions deemed acceptable.
A guy orders coffee at a new cafe and the Barista asks how many phone numbers he got last night. Guy says "Whoa! Outside the friend band yo!"
Bob and Sally are on a second date. Bob asks Sally "Hey Sally, what 'kinky' stuff do you like?" She replies "Our friend band hasn't stretched that far yet."
Best girlfriends of 6 years could say "Don't get offended, but its within our friend band for me to ask you this. Have you considered waxing your mustache?'