That thing is called "弹幕DanMu" (Barrage), which is a type of synchronized comment that appears alongside video playback. In this video, most of the comments are related to "taking a group photo" and "sending blessings". Perhaps because Chinese characters can convey more information in shorter sentences, this form of commentary is not as popular in other places, but it can actually be quite interesting. It provides an interactive experience that transcends time and space. On Bilibili, you can adjust the display area, font size, transparency, scroll speed, and intelligent prevention of blocking the main content for danmu. If you don't like it, you can turn it off.