The callback url is a URL your application handles that we send the user back to once they have authenticated. The basic flow is:
1 - Your app sends the user’s browser to the Intralinks login page using the oauth/authorize API.
2 - User logs in
3 - Intralinks redirects the user back to your application using the redirect URL you specify in your application properties.
For more detail please see our "Authentication" document in the Getting Started section of the developer's portal.