What's AuthorizationLevel.User and how to use it correctly?

I just configured my function app to use azure active directory. After that I wanted to deactivated the function level authentication by changing the enum at the HttpTrigger Attribute. Then I discovered that there is the possibility to switch to "User" AuthorizationLevel which is described as:

Allow access to requests that include a valid authentication token

After I deployed my Function I only get 401 unauthorized requests, so i switched back to Anonymous authorization level.

public enum AuthorizationLevel
    {
        //
        // Summary:
        //     Allow access to anonymous requests.
        Anonymous = 0,
        //
        // Summary:
        //     Allow access to requests that include a valid authentication token
        User = 1,
        //
        // Summary:
        //     Allow access to requests that include a function key
        Function = 2,
        //
        // Summary:
        //     Allows access to requests that include a system key
        System = 3,
        //
        // Summary:
        //     Allow access to requests that include the master key
        Admin = 4
    }

So my question is, how to use "user" authorization level correctly? And can I use it with azure active directory enabled?