"playing the slots" is simply a term used to describe two women having lots of oral sex.
it can be used in a variety of ways. a woman that is in a lesbian relationship (especially a new one) and suddenly has no time for her friends, could be described as "playing the slots", especially if she herself has never been in a lesbian relationship, hence the operative word "playing". another time when this term can be used is when a lesbian is hooking up with many different women, thus playing the slots. "playing the slots" is also something that straight or bisexual friends do when they get bored. when one asks the other if they want to "play the slots", it is generally out of boredom.