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.. so assigning a featured image to your posts is very important.
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.
Additionally, you can set style for your Featured Image withing page header in a metabox on the left side titled "Page Header". You can choose between Standard, Large, Fullwidth, Fullscreen styles.