Featured images are the image that will appear at the top of your blog posts or in the preview of your blog posts. If you are using the classic editor, they will also appear at the top of your pages.
Unless another sharing image is set inside e.g. Yoast SEO, it will also be the image that shows up if you share your blog post on e.g. social media. For pages, it will be the sharing image, if it is the first image displayed on the page and there is no other sharing image set in e.g. Yoast SEO.
This tutorial will show you how to set the featured image for pages, posts or any other custom page or post types such as events or portfolio entries.
Set the Featured Image
The video tutorial is using a blog post to visualize the steps, however, it will look the same on most other page or post types, including events or portfolio entries.