started by the Japanese video sharing site "NICONICO".
Users can upload, view and share video clips. Unlike other video sharing sites, however, comments are overlaid directly onto the video, synced to a specific playback time. This allows comments to respond directly to events occurring in the video, in sync with the viewer—creating a sense of a shared watching experience.
as you watch the video, you can see what other people are commenting or have commented about it at a particular point of the video's timeline. People would express thier opinions, feelings, excitement and fear, or warn you about something coming up. For exampe in horror movies, people would comment "alert" messages and even cover the screen with these comments so you can't see the scary scene. "comment shield"
then it expanded to China.
it can get pretty intense and annoying.
because these comments scroll from right to left, some people use the platform to make patterns and some form of art.
watching videos with the "bullet curtain comment" has become a neccessity for many people who are used to it. It makes the video watching experience more interactive.
example of this from a Chinese video-sharing website "bilibili"