If you download something using torrents, as you download it to your computer, you are also sharing it with others by uploading it. Once it's fully downloaded, as long as you leave the torrent program running on your computer, you will be "seeding" it, so if someone else comes long to download it they will, at least partially, be downloading it from you. If there are 100 people seeding it, if someone comes along to download it (which if I recall used to be called "leeching") they'll download it very fast. If there's one person seeding a concert and 100 people trying to download it, it will be very very slow.
Witchwood is correct, I think the one I used was uTorrent too, but there used to be several of these torrent servers to choose from. Torrentz was another one I think. It is a little daunting if it's completely new to you, but pretty easy to get up and running. Also, leaving it running and uploading after you've downloaded something used to be considered good form, but in the earlier days i ran foul of the old data limits internet companies used to impose if I accidentally left something uploading for days, so I'd be screwed and left with terribly slow internet for the rest of the month 😆
Edit: I assume leaving it seeding for a while is still considered good form, it's how the community shares.