During torrent download what it uploads?

Answer 1

The BitTorrent protocol, upon which torrent downloads rely, functions by sharing files between "peers" - lots of people running the same client. People with the whole file are called "seeders", and they will automatically share the file with others. People who haven't completed the file are called "leechers", and they'll download from seeders. When your torrent is uploading, you're sharing the parts of the file you already have with people who haven't got those bits. If you all keep uploading and downloading, you can assemble the file together completely. It's a good idea to let your torrent upload - you're being nice to other people who want the same file.