drupal module defines front page using hook_menu...What does it need to return?
Yes, if you return either a string or a render array from a page callback function, it will be rendered in the main content region on the front end (well, under normal circumstances anyway. There are possible exceptions).

There's an in-depth description of page callback in the hook_menu() docs which is a bit long to reproduce here. I'd recommend giving it a read through.