The tracker tells you who else has the files you want, in a nutshell.

Basic bittorrent outline:

You download a .torrent file which contains (amongst other things) a list of files, their hashes, and a list of trackers to connect to.

Your BitTorrent client connects to the tracker and says "Hi, who else has or is looking for these files"?

It returns a list of these people (aka "peers") and your BitTorrent client then connects directly to peers to download the files (and upload them to other people).