They've known each other since they were teeny tiny, and when Winnie's big brother died serving in Vietnam Kevin kissed her. Then they were friends, etc etc, and then in this episode they started getting closer again, but Winnie starts attending another school and getting involved with questionable kids, wanting her distance from Kevin but also missing him. It becomes clear that she's doing this because she wants to forget her brother and everything involving that sad part of her life. She tells Kevin to leave her alone, but shortly thereafter she's involved in a car accident involving her new friends. Cue clip.
Years and years later, come the end of the show, they have been on and off seriously for a long time. The show finale, set during their college or late highschool years, is a series of bad things, resulting in them realizing that they love each other but should not be together. Kevin's voice-over narration then reveals how all of the characters end up, concluding with Winnie, who, five or so years later, returned from an extended stay in Paris. Kevin was there at the airport to meet her, with his wife and child. The above clip and the finale are the only times in the show that they say "i love you."
if you find all that unbelievably corny, i apologize. it has always seemed very honest to me.