WordPress at a glance
filter is not described

wp_get_nav_menus filter-hook . WP 3.0.0

Filters the navigation menu objects being returned.

Usage

add_filter( 'wp_get_nav_menus', 'filter_function_name_4348', 10, 2 );
function filter_function_name_4348( $menus, $args ){
	// filter...

	return $menus;
}
$menus(array)
An array of menu objects.
$args(array)
An array of arguments used to retrieve menu objects.

Where the hook is called

wp_get_nav_menus()
wp-includes/nav-menu.php 590
return apply_filters( 'wp_get_nav_menus', get_terms( 'nav_menu',  $args), $args );

Where the hook is used (in WP core)

wp-includes/customize/class-wp-customize-nav-menu-setting.php 220
add_filter( 'wp_get_nav_menus', array( $this, 'filter_wp_get_nav_menus' ), 10, 2 );