A "lemon" yaoi scene refers to a graphic sex scene that goes all the way to actual intercourse, as opposed to a "lime", which does not involve penetration. A lime is kind of a build-up without the full-out release (oh, the participants release alright, but not during actual sex). Builds toward sex, but never quite reaches it; often the people are interrupted by something.

Wow, that was nice and graphic, huh? I hope that made sense. ;>

Cheerio, dear!