Usually, friends with benefits (a.k.a. FWB) means that people who know each other engage in intimate/sexual activity without really dating each other. Its different from hooking up, which tends to be a one-time thing with someone new.

What do guys really think of their female friends?

Men report more sexual interest in their female friends than their female friends do in them, and men are also more likely than women to overestimate how romantically interested their friends are in them. In most cases, sexual attraction within a friendship is seen as more of a burden than a benefit, the study finds.