A torrent tracker is a server which helps to transfer files between peers using the BitTorrent protocol. First, a peer informs the tracker about its interest in a torrent. The tracker keeps record of the number of seeds, peers, etc. So it responds in its turn with a list of other peers available. Once you’ve downloaded a .torrent file from any Mac torrent tracker and added it into your torrent client, the latter will send requests to other peers to get portions of that file.
Trackers can be public, or open, and private ones, which require authorization.