It's when you start "talking" with a girl. It's the beginning of "part x.xx", which is similar to the "bases" for relationships. The closer you get with the girl, the higher the number. If you start going out with the girl, that turns into part 2. One night stands can be as high as part 1.99 since you're not actually going out with the girl.
Brian: I heard that "G" got up to part 1.99 with some guy. Wtheo!
Wes: Yeah. Dude, she's not going out with that guy, but they got pretty far.
Brian: I got to part 2 already. Even up to part 2.8, woohoo.
Brian: I haven't gotten to part 1 because of how long I've been in a relationship. Yee.