It’s a meme where someone issues a threat against someone else, then veils it against moderation & reporting by appending “in Minecraft.” e.g. a user responds to your post with “I’m gonna burn your house down”, which can be construed as a real-world douchebaggery. But if they say “I’m gonna burn your house down in Minecraft”, there’s plausible deniability because it’s infers online douchebaggery where real physical harm is unlikely.