The top of a soda/pop/coke can given to someone indicating that the giver would like to fuck the receiver. If the receiver returns it to the giver, the giver owes them a sexual favor.
Jane: It's a fuck tab, Jamie.
Dick: Hey, Jane *gives her the tab* In my car after school?
Jane: Sounds great!
Child: "Mommy, why do you have all these coke tabs in a jar?"
Mother: "Oh...no reason..." *calmly takes jar from child and hides it*