Im scrolling through forums and I just see that. Does that really mean anything?

On wiki sites like Wikipedia, when you add information to an article, you are expected to cite where you got that information from.
Making unsourced claims often leads to that information being removed, as it is not possible to verify that the information is true.

Sometimes, if an unsourced piece of information is widely believed or known to be true, it will remain as-is, but with a [citation needed] note placed beside it.
This is to indicate that the preceding information does not have any source to back it up, and that an editor should try to find one.

On the Scratch forums, this has become a sort of in-joke with some users. When someone makes a claim without proof, another user might reply with [citation needed], often followed by a rebuttal or request for proof.

thanks

That is usually used on Wiki pages when something is unconfirmed to be true.
We use it on Scratch as a little inside joke when saying something is commonly/mostly believed to be true, but is not confirmed by the ST.