Well some trackers (the ones you sign up to) insist that you upload at least as much as you download to be fair to others.
When you are downloading you are uploading too and if it was in step you'd have a ratio of 1.00 or 100% whatever.
However your upload speed is MUCH slower than your download so in practice after you get the film you must leave it uploading for others for a fair while until it reaches at least 1.
You can copy it to a different part of your drive so you can use it, but leave it uploading as long as you can.