An application SID is the unique identification for a TwiML App; a collection of urls and configuration options. Inside a TwiML App, you can specify URLs for handling your Twilio Programmable Voice, Fax, and Messaging communications, as well as URLs for status callbacks and fallbacks. TwiML Apps are required for using Twilio Client in-browser calling.

Using TwiML Apps instead of individual URLs can also make managing multiple phone numbers easier. Users can simply update URL(s) in a TwiML App, rather than having to update the configuration for each of your phone numbers.

Users can create and manage TwiML Apps from the voice and SMS sections of Twilio Console:

• Programmable Voice: https://www.twilio.com/console/voice/twiml/apps 
• Programmable Messaging: https://www.twilio.com/console/sms/runtime/twiml-apps

For help managing TwiML apps via the REST API, see REST API: Applications (Twilio Docs).