Final answer:
A callback URL in a Salesforce connected app is where Salesforce redirects the user after authentication, providing the app with the necessary tokens to begin a session.
Step-by-step explanation:
In Salesforce, a callback URL is an endpoint for a connected app that Salesforce calls back to after completing the authentication process with the user. When building or setting up a connected app, you are required to define the callback URL within the app settings. This URL is where Salesforce redirects the user with an authorization code (if using Authorization Code Grant) or access token (if using Implicit Grant) which the app then exchanges for an access token to start a session with the user.
For example, if you have a web application that uses Salesforce as an identity provider, once the user logs in through Salesforce, Salesforce will send them back to your web application by redirecting to the callback URL you have configured. It's an integral part of the OAuth 2.0 protocol, being the link that completes the authentication flow by enabling communication back to the app after Salesforce has authenticated the user.The callback URL is essentially used to send the authorization code and/or access token back to the application after the user has granted permission to access their Salesforce data.For example, if you are building a web application that integrates with Salesforce, you can specify the callback URL where Salesforce will redirect the user's browser after authentication.