To use the Tropo WebAPI, your code must be on a web server accessible to the public Internet. Often developers find it quicker and easier to work on their local development machines, but these are usually behind firewalls, NAT, or are otherwise not able to provide a public URL. You need a way to make your local web server available to the Internet.
A tunneling service allows you to expose specific ports and services on your development computer as a public URL, even if your IP address is constantly changing or is shared by a huge number of people. Your computer connects out to the service, the service gives you a public hostname and port, and when you access that hostname and port, the service forwards this request over the connection your computer has opened.
There are many tunneling services out there, ranging from commercial products to open source tools you install on your own web server, to instructions on how to use your shell's SSH command to create a tunnel with a public server you control. A popular commercial service for this is ngrok. Ngrok doesn't have any connection to Tropo, but is well used by our staff and our developer community.