Implying that there is a list of boxes that can be checked in life, indicating the various experiences one has had, and checking off one of them. Almost exclusively in reference to sexual experiences. One's sexual resume. Various skin colors, sexual orientations, and physical features are all examples of boxes that can be checked off one's list.
I don't know if I'd do her (the shaved bald, heavily pierced, 200 lb woman across the way), but if you want to check that box, go right ahead.
Man, I'd really like to check that box (referring to the slim Asian hottie friend-of-a-friend).