Show the pattern: "have_posts() checks if there are posts left, the_post() advances to the next one and sets up global post data. Inside I use the_title(), the_content(), the_permalink(). After a custom query, I call wp_reset_postdata()."
The Loop is the PHP code WordPress uses to iterate over queried posts and display them. The basic structure: if (have_posts()) while (have_posts()) the_post() — this sets up the global $post variable for each iteration. Inside the loop, template tags like the_title(), the_content(), the_excerpt(), the_permalink(), the_post_thumbnail(), and the_date() output post data. The loop works with both the main query and custom WP_Query instances. Strong candidates explain: the difference between the main loop (driven by the URL) and secondary loops (custom WP_Query), why wp_reset_postdata() is needed after a secondary loop, and the difference between the_title() (echoes) and get_the_title() (returns).
Fundamental WordPress concept. Candidates who cannot explain The Loop will struggle with any theme development. Ask them the difference between the_ functions and get_the_ functions — it reveals whether they understand echoing versus returning.