WordPress at a glance
action is not described

woocommerce_after_checkout_validation action-hook . WC 1.0

Usage

add_action( 'woocommerce_after_checkout_validation', 'action_function_name_8116', 10, 2 );
function action_function_name_8116( $data, $errors ){
	// action...
}
$data
-
$errors
-

Where the hook is called

WC_Checkout::validate_checkout()
woocommerce_after_checkout_validation
woocommerce/includes/class-wc-checkout.php 878
do_action( 'woocommerce_after_checkout_validation', $data, $errors );

Where in WP core the hook is used WooCommerce

woocommerce/includes/class-wc-cart.php 112
add_action( 'woocommerce_after_checkout_validation', array( $this, 'check_customer_coupons' ), 1, 2 );