What is seeding?

http://computer.howstuffworks.com/bittorrent.htm

Simply, seeding is allowing uploads to continue after you have received 100%

With 100% and sharing, you are now a seeder.

Bittorrent is peer to peer network. This means that you download not from a server but form other people like yourself. Seeding is very important, in this way you let others download files, like someone did for you.