Post meta is the system WordPress uses to store arbitrary key-value pairs of data associated with a specific post, page, or custom post type entry — extending the fixed columns of wp_posts with unlimited custom fields. Every piece of post meta consists of a meta_key string, a meta_value (which can be a scalar or a serialized array), and the post_id it belongs to, stored in the wp_postmeta database table. The WordPress Metadata API provides the primary functions: add_post_meta() to insert, update_post_meta() to update or insert, get_post_meta() to retrieve, and delete_post_meta() to remove. Post meta powers Custom Fields visible in the WordPress admin, as well as most plugin data: Yoast SEO stores title and description overrides as post meta, WooCommerce stores product prices and SKUs as post meta, and Advanced Custom Fields stores all its field values as post meta.
Glossary entry