WordPress at a glance
function is not described

register_post_meta() WP 4.9.8

Registers a meta key for posts.

No Hooks.

Return

true/false. True if the meta key was successfully registered, false if not.

Usage

register_post_meta( $post_type, $meta_key, $args );
$post_type(string) (required)
Post type to register a meta key for. Pass an empty string to register the meta key across all existing post types.
$meta_key(string) (required)
The meta key to register.
$args(array) (required)
Data used to describe the meta key when registered. See register_meta() for a list of supported arguments.

Code of register post meta: wp-includes/post.php VER 5.1.1

<?php
function register_post_meta( $post_type, $meta_key, array $args ) {
	$args['object_subtype'] = $post_type;

	return register_meta( 'post', $meta_key, $args );
}