What is a Featured Image?
A Featured Image is a representative picture for your post which is used in a number of areas & functions, like on the homepage, post page, related post areas, recent post widgets, featured area sliders, etc..
How do I assign a Featured Image?
To assign a Featured Image to a post, navigate to your post's create/edit screen and look to the bottom-right. Here, you'll see a box titled "Featured Image" where you can add your desired image. Be sure to update your post when you're finished.