Seeding is how torrents are shared. Once you download a torrent and it is completely done, it will start seeding. That means that your computer will make the file available to other torrent downloaders for them to download some of the torrent from your computer. The more seeders you have, usually the faster the download. Someone who is downloading a torrent is called a Leacher. While this might sound like a dangerous thing, other computers can not access your computer in any way from you seeding a file.
It is very unethical to download torrents but never seed. Without people seeding torrents, it makes it hard for other people to download the torrent because there is no one to download it from. So after you torrent something, you should leave it in the list as "Seeding." After a while you can remove it but at least leave it there for a while. Note that if you change the name of any part of the torrent you downloaded, the client won't be able to identify it anymore and the seeding will stop.