Torrent trackers is a server that communicates between two peers to transfer file by using BitTorrent protocol.Once you have downloaded a .torrent file from the any torrent sites like the pirates bay or kickass torrent and added it in your torrent client software, it will request the file that was downloaded on other PCs. where the PC is called peers. A portion of the file that was download from a peer will be sent to you to download.
Trackers will always keep a complete record of the torrent like the number of leeches, seeds, peers, size, files that are in the torrent, and a number of downloads. If we add more public or private Torrent tracker, Torrent client communicate with those torrent tracker to increase the speed of the download by updating more addition connection with the peers.
Looking for a list of private torrent trackers? Check here