in simple terms, a private torrent site (that is closed to public). only users with an account can log in and use the torrents in the website.
each torrent that the users use has a unique passkey (consider it as an id) so that the site will know what torrents you have downloaded and how much you have seeded it back. they use these stats to monitor users, checking whether they seed for a certain amount of time and whether the users have the required ratio. why do they do this? to make sure that all torrents in their site is available to everyone (meaning there will be no torrents that has 0 seeders, rendering the torrent not being able to use anymore since no one seeds it).
there are guides out there, check out !trackers , maybe they might have a guide.