When someone accepts a person's friend invitation on facebook regardless if he/she even knows or likes the person, mainly accepting the invitation so it makes his/her friends list bigger.
+1 refers to the fact that it adds 1 more person to their friends list.
Guy 1 - Dude, Steve has so many friends on facebook! How is he so popular?!
Guy 2 - Most of them are +1s, out of the 300 friends he has he probably only knows 80 of them.
Guy - "(insert name here) wants to add me? I don't really like him but what the hell +1 I suppose."