YouTube has started testing a new feature that gives you a quick way to view comments "timed to the exact moment you're watching in a video" on mobile. The website has long allowed you to link to any part of a video you want simply by typing the current timestamp in your comment. As Android Police notes, though, this experiment gives YouTube a feature similar to SoundCloud's, where users are able to see comments tied to specific parts of a song as tick marks on the scrobble bar.
Also, it sounds pretty similar to how the replayed chat section in a YouTube Live video works if you watch it after the stream is done. The feature could help you find the most exciting or coolest parts of what you're watching based on what people are saying. Since timed comments are just an experiment at this point, though, YouTube has only enabled them for some videos and has only given access to a small number of people. Whether or not the feature gets a broad release depends on the feedback the team receives from the testers.
To know if you were chosen as a tester for the experimental feature, pull up a video's comment section on Android or iOS. Tap on the Sort button on the right—hand part of the section and check if you have the "Timed Beta" option along with "Top comments" and "Newest first."