A hook is a point in the theme where you can insert anything you want very easily. For example a HTML alert or a Google tag manager.
Available hooks so far:
wp_head();
header.php, bs Preloader hooks herewp_body_open();
header.phpwp_footer();
footer.php, bs Cookie Settings hooks here
Usage
Just insert this in your child theme funtions.php:
function my_function() {
echo '<div class="container mt-5"><p class="mt-4 alert alert-info">Hello Hook</p></div>';
}
add_action('wp_body_open', 'my_function', 5);