Theme Customizer Action Hook

Hi everyone,
I was going through theme TwentyFourteen files and stumbled into the “back-compat.php” file where a strange action hook was used to prevent the theme customizer from being loaded.

add_action( ‘load-customize.php’, ‘twentyfourteen_customize’ );

I was trying to understand what’s going on here, but I could not figure out where this action hook was defined!! I could not also find it any where in the documentations.

In this a new action hook, or is it a plugin file?
How does it get triggered?

Thanks