Impersonating a user means providing the administrator access to what the actual user would have access to. This will have the same modules as well as menus. ServiceNow records all activities of an administrator when the user is impersonating another user. This feature of ServiceNow is very helpful in testing. For example, if you want to test that whether a user is able to access the change form or not. You are allowed to impersonate that user and can perform testing without any need of logging out from your session and again logging in with that user credentials.
