Keep it simple: "update_post_meta to save, get_post_meta with true for a single value. I prefix keys with the plugin or theme name to avoid collisions. For querying, I use meta_query in WP_Query but keep in mind it is slower than taxonomy queries."
Post meta (custom fields) stores key-value pairs associated with a post in the wp_postmeta table. Store with update_post_meta($post_id, $key, $value), retrieve with get_post_meta($post_id, $key, true). The third parameter (true) returns a single value instead of an array. Delete with delete_post_meta(). Meta can be any serialisable value — strings, numbers, or arrays (WordPress serialises them automatically). Common uses: storing extra data like event dates, product prices, or custom settings that do not fit standard post fields. Strong candidates mention: that meta keys starting with underscore are hidden from the Custom Fields UI, the meta_key and meta_value parameters in WP_Query for querying by meta, and that ACF and similar plugins store their data as post meta.
Fundamental WordPress data storage concept. Candidates who cannot explain post meta will struggle with any custom data requirements. Those who understand the API and the performance implications of meta queries versus taxonomies make better architecture decisions.