The term 'lemon' pops up in older corners of fandom spaces, and it's basically a straightforward content warning. It refers to fan stories that contain explicit sexual content. I think its origins are kind of vague, but I've always heard it came from anime fandoms back in the 90s, maybe from the 'lemon party' meme or something like that. The opposite would be 'lime' for less graphic or suggestive content.
It's one of those labels that helps readers filter what they want. If you're browsing a fic archive and see 'lemon' tagged, you know what you're getting into. It's fallen out of favor a bit in some newer communities that use more direct tags like 'Explicit' or 'Mature', but you'll still see it around, especially in older fandoms or on specific platforms. Honestly, the terminology feels a little dated now, but it's part of fandom history.