In connection with torrents, seeding is the term for when you have completed downloading a torrent or a piece of a torrent and are now completely sharing by uploading that data to other users who are downloading the same file.
Most private torrent sites (site you need to register with) require that you keep a 1:1 (download:upload) ratio, to remain in good standing, and that is only done by seeding the file or files after you are done completely downloading them.
In public torrent websites (registration not necessary), many users do not feel the need to seed and therefore other users suffer slow download times.
Seeding is the foundation of torrent downloading.
Related glossary terms
Related software tools
µTorrent (aka uTorrent) is an efficient and feature rich BitTorrent client for Windows.
Azureus is Java based BitTorrent client, with multiple torrent downloads, seeding, embedded tracker and lots more.