uTorrent works by downloading files from other peoples' computers. In order to do so, other people need to share those files. The red bar means that no one is sharing that file at the moment, and so you can't download it.
From their help site: http://www.utorrent.com/faq/#faq28
28. What do the colors in the Availability graph mean?
•The color red means the piece is not available amongst the peers you're connected to. (this means availability is less than 1 and you may not be able to complete the torrent)
•Pale blue means that the piece is present amongst the peers you're connected to, but rare.
•The darker the blue, the more common the piece is in the swarm.
32. What does availability mean?
That number tells you how many unique copies of the file are available between yourself and the peers you're connected to. If this is less than 1, you most likely will not be able to complete the torrent. The only thing you can do is wait and hope a seeder hops onto the torrent to allow you to complete it. You can also try and request a "reseed" on the tracker's forum. Sometimes, this may never happen because the torrent was abandoned, and you may need to find the torrent elsewhere.
[ PEER: A peer is another computer on the internet that you connect to and transfer data. Generally a peer does not have the complete file, otherwise it would be called a seed. Some people also refer to peers as leeches, to distinguish them from those generous folks who have completed their download and continue to leave the client running and act as a seed.
SEED: A computer that has a complete copy of a certain torrent. Once your client finishes downloading, it will remain open until you click the Finish button (or otherwise close it.) This is known as being a seed or seeding. You can also start a BT client with a complete file, and once BT has checked the file it will connect and seed the file to others. Generally, it's considered good manners to continue seeding a file after you have finished downloading, to help out others. Also, when a new torrent is posted to a tracker, someone must seed it in order for it to be available to others. Remember, the tracker doesn't know anything of the actual contents of a file, so it's important to follow through and seed a file if you upload the torrent to a tracker. ] ( http://btfaq.com/serve/cache/23.html )
See http://en.wikipedia.org/wiki/BitTorrent_(protocol) for more.