WordPress has the ability to assign a featured image to posts and pages.
A featured image is an image from your media library that has a special meaning to your WordPress theme.
Each theme styling has a different way of presenting the featured image. Some themes will only show it on the single post or page, others will show it on summary and archive lists too. Some themes will present a large image, others just a thumbnail. Themes like TwentyTwelve will actually use the featured image as the header background, but only if it is large enough. The best way to find out what featured images look like in the theme you are using is to try one out.
Adding a featured image is simply a matter of editing the post or page and scrolling down (or across) and finding the ‘featured image block’