With some of my earlier girlfriends, I would practice this act a lot with them before we got "serious" and would have sex regularly. It involves the female lying on top of the male, and positioning her vagina on the penis, WITHOUT penetration, and rubbing it up and down the length of the cock, producing stimulation of the penis, and usually actually stimulating her clitoris, as well.
If you are having trouble with my description, the 3rd movie on this website has a textbook example of it.
It's called frottage. The word derives from French, and rhymes with fromage. You can do it with clothes on as well, even in public places, if you are clever.
Rubbing genitals together is sometimes referred to as tribadism when it involves lesbians.
I know a couple who like doing it at dinner parties. It is amazingly erotic (but frustrating) to watch. I also saw them do it on a crowded train one day. There were a few stares from the other passengers, but no-one complained or tried to stop them.