The Twilio Programmable Voice JavaScript Quickstart Demo Application allows users to make and receive calls via their web browser using the Twilio Voice SDK 2.0. The application requires a Twilio account, Node.js version 14.0 or above, ngrok, and a WebRTC enabled browser. Users can create a TwiML Application, purchase a phone number, and generate an API key to authenticate with Twilio. The application provides a ready-made starting point for writing voice apps and includes features such as device registration, connection options, and demo app customization. To use the application, users must configure their TwiML app, expose their application to the internet using ngrok, and make outgoing calls by entering phone numbers in E.164 format. The application also handles incoming calls from non-browser devices and provides a way to customize device settings such as edge location and sound options.