TikTok has a feature called “Duets” that allows users to place their video alongside (or above) an existing video.
In theory this allows them to sing duets together and harmonize beautifully with their friends and idols and so forth. In practice it’s either used to harmonize badly or to parody and mock the very concept of cringe-inducing duets. That said some uses of the feature are actually funny and creative and celebrities like Jason Derulo have jumped on the trend to post videos in which they react to their fans’ antics.