It's based on the idea of sexuality being a sliding scale, where 0 is totally straight and 6 is totally gay, and everyone is at some point on that scale. It supports the idea that sexuality is fluid, and that most people don't completely swing one way or the other.
It's a scale to determine sexual orientation. It ranges from 0 to 6, 0 being completely heterosexual and 6 being completely homosexual, 2-4 being bisexual and 1 and 5 being bi-curious.
