Uploading data (as opposed to downloading) takes up much more bandwidth in comparison, so you need to keep programs like uTorrent from uploading too quickly. Left unchecked, uTorrent can use all of your bandwidth, rendering your connection unusable for any other purpose. By disabling its ability to upload you'll keep a fair amount of bandwidth open. However, peer-to-peer networks rely on their users to share the data they download; excessively downloading without uploading is a quick way to get blacklisted.
Click the "Options" menu and select "Preferences."
Select "Bandwidth" and find "Number of upload slots per torrent." Change the amount to 0.
Click "Apply" and then "OK" to save your settings.