When uTorrent, or whatever Torrent application you use, finishes a download, it starts "seeding" meaning it is uploading the video, or whatever you are downloading, so others can download it faster.

You also seed automatically when you are still downloading the file.

If you are downloading a movie and it is on 76% then you will be seeding the parts you have downloaded so far as well as downloading it at the same time.

You can limit your upload speed to a certain amount.
Sometimes when seeding it slows the speed internet pages load up so you can limit the upload speed to a smaller size so you wil still be seeding but will still have good internet connection.