not much to know. it’s a distributed download
it goes like this:
someone hosts the tracker, the tracker keeps track of the swarm, when a new person connects to the swarm he sends out “i have no copy of this, please give” the swarm responds by sending tiny pieces of the file. your client keeps track of what goes where and gradually assembles the file. when it receives every piece, it says “thanks” and then goes into a seeding mode where it prepares to send pieces of the file to others, helping anybody new.
think of it as downloading from 100 servers piecemeal, instead of one. saves alot of bandwidth for whoever’s providing the file. sharing is caring.