WordPress at a glance
action is not described

add_meta_boxes action-hook . WP 3.0.0

Fires after all built-in meta boxes have been added.

Usage

add_action( 'add_meta_boxes', 'action_function_name_3366', 10, 2 );
function action_function_name_3366( $post_type, $post ){
	// action...
}
$post_type(string)
Post type.
$post(WP_Post)
Post object.

Where the hook is called

register_and_do_post_meta_boxes()
wp-admin/includes/meta-boxes.php 1416
do_action( 'add_meta_boxes', $post_type, $post );
wp-admin/edit-link-form.php 34
do_action( 'add_meta_boxes', 'link', $link );
wp-admin/edit-form-comment.php 181
do_action( 'add_meta_boxes', 'comment', $comment );

Where the hook is used (in WP core)

Does not used.