A localhost tunnel establishes a connection between your local machine and a remote connection. The connection is intended to proxy traffic from a publicly-addressable IP address and URL to your local machine. Localhost tunnels are most useful for allowing a tester to connect to a server running on your local development system so they can try out an in-development application you are building but have not yet deployed.
There are numerous localhost tunnel services that have similar features. The following services are listed in order from ones I have had the most experience with to the ones I have not used.
ngrok is the service I use most often. It is easy and worth the small fee to upgrade your account with a few extra features such as fixed, customizable subdomains.
Localtunnel is a localhost tunnel written in Node.js.
Burrow provides another service, albeit one that I have not used myself.