Genderfluid? What does it mean?

So my boyfriend told me he's genderfluid, he briefly explained some of it to me since he had to go to work. He said he's not gay, but his mind sometimes shifts from male to female based on the sexual situation and how he expresses himself. I don't know if it means for in general too, I have yet to ask that, I don't think so. He said he of course still wants for us to be in our relationship, and nothing has changed it after him telling me this since I'm trying to understand. He didn't see why I would ask that question, but I told him I'm just trying to gain a clearer idea of what he means. His mind hasn't shifted with me from male to female. We're in a polyamorous relationship. We're talking more about him being genderfluid later after he gets out of work. He has been this way for over a year, which was way before we got into our poly-relationship, but we've known each other all our lives. I don't mind if he's bi either by the way. I just always thought there was gay, lesbian, transgender, and transexual... Can someone please explain this to me a bit more clearly so I don't fall down the endless rabbit hole of Google searches?