Seeding is where you leave your BitTorrent client open after you've finished your download to help distribute it (you distribute the file while downloading, but it's even more helpful if you continue to distribute the full file even after you have finished downloading).
Or in simpler terms
If you have a download 100% completed you become a seed.
BitTorrent will keep uploading (seeding) to other users so they can also complete their download.
Keep in mind: If nobody would seed you could never complete a download.
So please leave BitTorrent open seeding as long as you can to help others.
Hope this helped