A meta box is a draggable box displayed in the post editing screen in the backend of WordPress. Users can select or enter extra information in meta boxes addition to the content in the main post editing area.

There are two types of data you can enter in meta boxes: metadata (i.e. custom fields), and taxonomy terms.

1. Adding a Meta Box

WordPress provides a add_meta_box function with the specific purpose to add a new Custom Meta Box. add_meta_box has to be called from inside a callback function that should be executed when the current page’s meta boxes are loaded. This task can be performed hooking the callback to the add_meta_box_{custom-post-type} action hook, as suggested in the Codex.

That being said, let’s add the following code to the main file of a plugin or a theme’s function.php file (keeping in mind that it’s always best to create a child theme instead of altering a functions.php file):