A date buffer is a friend that you bring along when you hang out with someone that you're romantically interested in. The purpose of this is to make the excursion seem as if it wasn't a date, but more of a friendly gathering. In most cases though, the 'buffer' is ignored as a third wheel.
However, if you're NOT interested in the person, a date buffer can be used so that you don't send the wrong message.
1) "Dude, I wanna go out with that chick again, but I'm a wuss and I don't wanna go alone. Could you be by date buffer?"
2) "That guy's cool and all, but I don't really like him like that. Just be my date buffer today and I'll pay for your ticket."