Bubbling of hooks and filters

Hello,

Let me introduce my self, my name is Simon I’m PHP/AJAX/XHTML/CSS programmer.

I’m newcomer in the magical world of WordPress

Turn to the concrete:

WordPress 2.5.1 version provides a default theme, located in the folder https://site/wp-content/themes/default

Just take a look at: functions.php

1.
      function kubrick_header_image() {
   2.

   3.
      return apply_filters('kubrick_header_image', get_settings('kubrick_header_image'));
   4.

   5.
      }

and there is where I’m confused:

• is “kubrick_header_image” is a hook that already exists ?
• and what is the callback function in this case? Where is it located?

Friendly

Simon