It's an osu! forums version of that one forum game on Kongregate's forums you see all the time. Basically, what you do is you are given a random thing, specified by the user in the post above. You say it's actually something that looks like/is related to the thing above. Then you pass the new thing onto the user below; if they ever post, because new forum games eventually go inactive, right? Anyways, the user does the same thing as you did before. That's not this, that's that, pass it on, blablabla, and that's it.
Example
Person 1: The starting object is a PC.
Person 2: That's not a PC, that's a Mac.
Person 3: That's not a Mac, that's a Linux computer!
Person 4: That's not a Linux computer, that's Tux!
Person 5: That's not Tux, that's a normal penguin!
Person 6: That's not a normal penguin, that's the penguin from Surf's Up!
Again, this continues forever, as Person 6 passes it on to Person 7, who is not seen in that example.
Person 2: That's not a PC, that's a Mac.
Person 3: That's not a Mac, that's a Linux computer!
Person 4: That's not a Linux computer, that's Tux!
Person 5: That's not Tux, that's a normal penguin!
Person 6: That's not a normal penguin, that's the penguin from Surf's Up!
Again, this continues forever, as Person 6 passes it on to Person 7, who is not seen in that example.