What is a SEED or SEEDING?

A SEED: A computer that has a complete copy of a certain torrent.
A SEEDER: A person who is sharing that file, by leaving there client running.
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.
So the key rule here is, when you download a file using Bittorrent, you need to ensure you keep the client/download screen open and DO NOT click the FINISH/CLOSE button when the download has reached 100%/COMPLETE.
If you do, then you STOP seeding the file, (sharing) with other peers (users).