It's an informal way of agreeing with/emphasizing the parent comment.

Your question isn't stupid. Using "this" in such a manner is non-standard English and I can see how the meaning would be unclear if English isn't your first language. It might be unclear even to native speakers if they don't spend much time on internet forums.

It means "I agree with this, but have nothing else to contribute to the discussion, and don't understand that an upvote would suffice, or I just want to pad the number of comments I'm making on this site."