Torrenting is distributing technology.
One person initially "seeds" a file.
That is tracked by a tracker which logs this persons level @ 100%.
If the tracker is announced, then others can connect to the tracker and find the seeder. As they download the file to 100%.. they are known as leechers or peers. The moment they download a single bit, if there is another than needs that bit, it will be shared. That's why the more downloaders, the better. More peers who are uploading bits and more seeders who are uploading bits.
Once they have 100%, if they leave the app open.. they become seeders as well as the initial seeder. Anyone connecting to the torrent can connect to the seeders and peers for "bits" of the file (until they reach 100%).
Torrent is depending on sharing. Really sharing.
If everyone doesn't bother to seed upon 100% download then it's going to be slow.
It's mechanism prevents those who just logon to grab a download and then log off.. which is bad etiquette (If you are into P2P sharing). The moment you have downloaded a bit.. you are a peer and can upload that to someone else. No fake sharing, no downloading without uploading. Fairer in P2P terms.
But if they dont seed and the initial stops seeding as well.. then the torrent will die.