It's software that runs a self hosted virtual router. You can use to access self hosted services over clear net if VPNs and Tor not cutting it or expose a service such as website to the public without exposing your home IP. https://docs.start9.com/start-tunnel/1.0.x/

https://docs.start9.com/start-tunnel/1.0.x/architecture.html It’s essentially an alternative to tor for interacting with start9 services through clear net using your server and a vps you control. I have not used it to be clear.