The concept is simple pay money to promote your comment.
It's a little funny considering that Youtube's comment section is notoriously bad. I don't see how paying to promote a comment will make the platform better overall.
From the Youtube Blog:
Anybody watching a live stream can purchase a Super Chat: a highlighted message in the chat stream that stands out from the crowd to get even more of your favorite creator’s attention.
For creators, this means Super Chat does double duty: keeping their conversations and connections with (super) fans meaningful and lively while also giving creators a new way to make money.
Imgur Melee