Revolving around the core of technology
Tor is a service that hides the real IP address of a connecting client. Although the service is created to provide anonymity on the Internet, many individuals misuse it with malicious intent. For example, it is very common for someone to guess user id/password combination against your server and use Tor to hide their identity.
Some products, such as SynaMan, allow administrators to deny login attempts if an IP address belongs to a Tor node, adding an extra layer of security.