A file you use with a torrent program like Deluge, Transmission or uTorrent to download a larger file via torrenting. Torrenting is a peer-to-peer file sharing method where you get (and share) pieces of the file from many other users. Depends on there being other people "seeding" the file at the moment, and can take longer than downloading a file in one huge chunk but does have advantages. Such as being able to pause and resume downloads when you want, and the torrent program should have a speed setting that lets you set how much of your internet it's using.
Though just like downloading, be careful when torrenting, bad actors can use them to spread malware as well. Importing a good block list into the torrent program can help with that.