An application SID is the way that a Twilio application is identified. An application is a container for a set of URLs and configurations that you can use for a phone number or a group of phone numbers. Using applications instead of individual URLs makes it easier to update all of your phone number configurations at one time in the case that your application moves. You can specify an application for Voice, SMS or for both.
You can create and manage application SIDs from your Twilio Console.
For more information on applications, see our applications documentation.