Authorization decides what an identified person is allowed to see or do. It is like checking which rooms someone may enter after their identity has been confirmed.
In a project app, one person might be allowed to read a report while another can also change it. The app checks those permissions before carrying out the action.
Authorization is different from authentication: authentication checks who signed in, while authorization checks what that account may do. Apps should check permissions on the server, not rely only on hiding buttons on the screen.