It's a script (think of it as mod) for plug.dj which will allow you to add YT (YouTube) videos to your playlists with one button.
Plug.dj is having some trouble with their YT API Key (the thing that allows you to make YT related actions throughout the site, like searching for a song and adding the results to your playlists). Using this script, you will be able to add YT videos to your playlists again, even while plug.dj is dealing with YouTube's support to get this issue fixed.