You've seen versions of them in TikTok, FB live and most other video live-streaming products but it's worth a read into the origin of "bullet comments"
Bullet comments, or 弹幕 (“danmu”), are text-based user reactions superimposed onto online videos: a visual commentary track to which anyone can contribute.
Each comment is synchronized to an exact moment in the video, and will fly across the screen on cue on every subsequent replay. On particularly popular videos, they pile up so thick that they can cover the original entirely. The result is a viscerally social experience, like an opening night crowd at a movie theater that you can summon anytime.