WordPress at a glance
action is not described

register_sidebar action-hook . WP 3.0.0

Fires once a sidebar has been registered.

Usage

add_action( 'register_sidebar', 'action_function_name_9790' );
function action_function_name_9790( $sidebar ){
	// action...
}
$sidebar(array)
Parsed arguments for the registered sidebar.

Where the hook is called

register_sidebar()
register_sidebar
wp-includes/widgets.php 276
do_action( 'register_sidebar', $sidebar );

Where the hook is used (in WP core)

Does not used.