There are several ways to use it:
You can enter the list of the Youtube playlist urls you want to play. In this case only public playlists can be played. Each line should have one playlist url or playlist id (for illustration purposes we put some examples into the textbox which you can delete, of course)
Alternatively, enter the Youtube username which the playlist belongs to. In this case only public playlists can be played.
You can give more than one user name separated with commas (e.g. joe,mary) if you want to mix playlists from several users.
If the user name does not work for some reason then use the youtube channel id instead. Here's how to find it.
You can also authorize this site to view your Youtube account and then you can see all your playlists instantly, including your private playlists. Click here to do that, but note that Youtube demands verification for apps which access user data, so you may get a warning this app is unverified, because I don't bother going through the verification process.