A featured image is an image that represents a blog post, page, or custom post type. It will appear as a thumbnail when the content is shared on social media. The featured image for a blog post will also appear on the blog’s home page, category pages, tag pages, and archive pages, as well as in RSS feeds. Most themes support featured images.
You can set a featured image for a post, page, or custom post in the Featured Image meta box that appears in the sidebar of the edit window.