The “Q” stands for “Queer.” No, really.
I’ve also seen some other letters after the Q, in some descriptions of the “LBGT” or “LGBTQ” community. I think that there are additional combinations in the sexual identity/sexual orientation industry. Sort of like Facebook has 56 different gender possibilities. They are getting carried away with all of this, in my opinion.
You would think that last letter is covered by the previous three, but I guess they wanted to drive the point home.