Ngrok allows you to expose a web server running on your local machine to the internet. It's very handy for developing APIs, in particular when working with Twilio.
Follow the quickstart guide here.
First fire up your application, with rails it would look like this:
rails server 3000
N.B. You will need the port number to instruct ngrok
Once your server is running, all that's left is to expose the port to the internet with ngrok:
ngrok http 3000
Where http is the protocol and 3000 is the port number. Ngrok will display a UI in your terminal with the public URL of your tunnel and other connection information.
Ngrok provides a real-time web UI where you can introspect all traffic running over your tunnels. After you've started ngrok, just open http://localhost:4040 in a web browser to inspect request details.
There are plenty more configuration options, including custom domain names etc. For more details, you can check the ngrok docs https://ngrok.com/docs