A seeder is a person/client which has a complete file to share with the others users.
Seeding is the process of connecting to a torrent when you have a complete file
There are two ways to do this:
...by leaving your client open after the download completes. Once you have the entire file you become a seed, and the BitTorrent client remains connected to the swarm, sending to other users until you close it.
...by clicking on a torrent link (or opening a saved .torrent file) and selecting a filename of a file that has already completed. BitTorrent will check over the file and realize that it's already complete, and continue to connect to the tracker and serve as a seed.