These are a type of comments named "danmaku" (弹幕), originated from a Japanese video site Niconico. Under the player there's a button appearing as a TV icon with

These are a type of comments named "danmaku" (弹幕), originated from a Japanese video site Niconico. Under the player there's a button appearing as a TV icon with a 弹 character in it; click the button to toggle these comments.