A BitTorrent tracker is a peer-discovery service. It helps participating clients find one another so they can connect directly, accelerate downloads, and work around the network-configuration friction that can otherwise keep peers from meeting.
At this phase of the BitTorrent protocol, the tracker handles only that networking problem. It receives connection details and returns a directory of peers; it does not receive or inspect the files being exchanged. The peers later communicate among themselves to negotiate and transfer the data, so the tracker cannot determine what content those files contain.