This one is rather simple, it just renders a two-field (username/password) form which is targeted at the current URL.
ArrayA simple associative array with the two indexes 'username' and 'password' holding the information read by the driver or null if no information could be read.
The default should be quite useable through its CSS.
If you want to replace the form by some custom style, you can define the style- or page-element midcom_services_auth_frontend_form. If this element is present, it will be shown instead of the default style included in this function. In that case you should look into the source of it to see exactly what is required.