Source (Unfortunately not sure where to find the Reddit post this references)
Your goal is to think of something worse than what the above poster said. If you can’t think of something, you you reply with an image saying “100 parking spaces”
This raises the question of what happens if someone refuses to concede defeat. I guess for now, instead of posting the 100 parking spaces images if you can’t think of something worse, post it if you think the previous user’s worse thing is actually better than the thing above them. If three people agree that the above user’s post isn’t worse than the one above it, they lose.