WordPress at a glance
function is not described

the_guid() WP 1.5.0

Display the Post Global Unique Identifier (guid).

The guid will appear to be a link, but should not be used as a link to the post. The reason you should not use it as a link, is because of moving the blog across domains.

URL is escaped to make it XML-safe.

Hooks in function
Return

Nothing (null).

Usage

the_guid( $post );
$post(int/WP_Post)
Post ID or post object.
Default: global $post

Code of the guid: wp-includes/post-template.php VER 5.1.1

<?php
function the_guid( $post = 0 ) {
	$post = get_post( $post );

	$guid = isset( $post->guid ) ? get_the_guid( $post ) : '';
	$id   = isset( $post->ID ) ? $post->ID : 0;

	/**
	 * Filters the escaped Global Unique Identifier (guid) of the post.
	 *
	 * @since 4.2.0
	 *
	 * @see get_the_guid()
	 *
	 * @param string $guid Escaped Global Unique Identifier (guid) of the post.
	 * @param int    $id   The post ID.
	 */
	echo apply_filters( 'the_guid', $guid, $id );
}