15 seconds of someone in a helmet/mask dancing as if they were invisible to the rest of the room, then 15 seconds of everyone dancing, usually awkwardly and with some kind of partial costume and/or odd props. And usually a bit of partial "nudity" and perhaps suggestive hip movements.
But it's so ridiculous that even children are convinced of how silly it is. And that is why they love it too. So this may be the most innocent version of it on the internet. It makes it no less annoying however.