Each WordPress post or page has exactly one “Featured Image.” Depending on the template used by your theme, this image may appear at the top of your post, on the side, or not at all. On a page that aggregates your posts or pages (perhaps by category or tag), the featured image adds interest to the listing. If missing, it throws off the appearance of the page.
That is where the Default Feature Image plugin can be useful. If a page or post does not have a featured image, a default image will be assigned.
The default image is set up in the Media Settings on the WordPress dashboard: