A seedbox is simply a version of utorrent that runs 24/7 on a server. This server has quite fast speeds (100mbps). Here is a screenshot of one of my seedboxes’ stats. Go to your utorrent and click settings and stats to compare:

Here is the screenshot of the client you would be using (utorrent WEBui). Pay attention to the upload speed:

And that is pretty much it. A seedbox is basically computer that can upload at fantastic rates