From the documentation:

$function

 

(callback) (optional) The function to be called to output the content for this page.

In other words, this is the function that will be responsible for outputting content on your submenu page. You need to create it, give it a nice unique name, and then specify this name as the last argument into add_submenu_page:

public function wp2804_submenu_output_function {
    ?>
    <h1>here be the title</h1>
    <?php
} // end of wp2804_submenu_output_function 
add_submenu_page( 'themes.php', /* all the other stuff */ 'wp2804_submenu_output_function');

From the documentation:

$function

(callback) (optional) The function to be called to output the content for this page.

In other words, this is the function that will be responsible for outputting content on your submenu page. You need to create it, give it a nice unique name, and then specify this name as the last argument into add_submenu_page:

public function wp2804_submenu_output_function {
    ?>
    <h1>here be the title</h1>
    <?php
} // end of wp2804_submenu_output_function 
add_submenu_page( 'themes.php', /* all the other stuff */ 'wp2804_submenu_output_function');