0 = 100% Straight
6 = 100% Gay
1-5 = Varying degrees of bisexuality.
a 1 or 5 could also be considered just straight or gay, as you probably wouldn't find the same sex attractive enough to actually date, if you're that close to being straight (or vice versa for being gay). A 2 or 4 would be bisexual, with a preference one way or the other... and 3 would mean you find both genders completely equally attractive.
(We've used it in this thread like... dozens of times, how did you never see it? lol)