A "normal" download is sending a file from 1 computer (e.g. server) to 1 other computer (e.g. client).

An example of that would be an HTTP server like the one we are both using now, e.g. you (client) visit lemmy.ml (server) and it sends you back the index.html page your browser requests. That is a great solution when you have a page that must dynamically be updated and broadcast back the new information to plenty of clients.

BitTorrent is a protocol like HTTP but instead of having 1 computer sending to many other computers, ALL computers send the part they have, ALL computers request the part they are still missing. That's a different solution for a different problem, namely when a file is large enough and stable enough (does not change) that all the overhead is worthwhile.

So seeding implies having enough upstream bandwidth in order to help others who are still have missing parts. Note that most BitTorrent clients already have useful seeding defaults. Typically they'll let you seed (namely share file parts) even after you have downloaded everything up to a positive ration, e.g. 2/1 meaning that you will keep on sharing until you have uploaded about twice more than you downloaded.

Hope that helps!