It's a pill that changes your gender. Pink turns you from a guy to a girl, blue is the reverse. Basic lasts 12-24 hours, Extra Strength is 1 month, and Plus is permanent. Basic doesn't allow pregnancy, the other two do. Pregnancy makes any pink pill permanent. Purple pills make you a trans woman (Woman's body except you have a dick).
There are a few other variants, too. Resistance is temporary, but each time you orgasm you have a small but cumulative chance of it being permanent.
Breeder makes the chance of pregnancy 100%, makes you super horny, increases the power of your orgasms, and prevents you from orgasming unless a guy comes inside you.
There are several other less common variants, like Denial (prevents you from orgasming altogether), Clone (transform into a specific person), Forget (you forget either your entire previous life, or you forget that you were ever your previous gender), Bimbo (turns you into a dim-witted, busty, horny, uninhibited woman who will sleep with anyone and everyone. Sometimes this pill includes things like permanent makeup or STD protection), or others.
Really, the sky's the limit, creatively. But the core idea is gender transformation mixed liberally with sex and sexuality.