Okay let's say you have a file. It's a video called 'I hate homework."
Then people all over the world also has this video.
So if everyone creates a torrent file of that video, you'll get a file called 'I hate homework.torrent"
Basically a torrent file is like a map. You need a software called torrent client to 'read' that map. Utorrent is an example of a torrent client.
So if you upload the 'I hate homework' video torrent to popular torrent sites like PirateBay.org, people can download that 'map'. Then they can download that video using that 'map' read by the torrent client.
The actual data (video data) is downloaded from people all over the world who has that exact video with the exact name (I hate homework) who has uploaded their video to torrent sites.
Torrent works by downloading and exchanging data with all other people who are either downloading or uploading that file. They do this by breaking the file into pieces.
E.g. I have this piece number 4567, I need that 8375. Then your torrent client will exchange that with another torrent client from another part of the world from another user. All this is done automatically by the software.
Seeders are basically people who HAVE that file. Imagine people planting vegetables.
Leechers are people DOWNLOADING that file. Imagine people like worms eating those vege.
Once seeders become less and less, there are less vege. So more and more leechers come, they eat up all the remaining vege.
Result: everybody starves. No more vege.
So seeding is important.