A troll is someone not using the platform in good faith, but instead trying to fish some type of negative reaction or result.
On a social media platform like Lemmy, a troll could be someone creating fights, baiting people with insults or fabrications, spreading misinformation purposely, adding links to content that might dangerous or illegal, and so on.
In short, a troll is someone engaging with others with the goal of ruining their experience. In a videogame it could be a team member losing on purpose to annoy others, for example.