Featured Image is the image intended to represent and make your post stand out. It’s shown in multiple places on the blog, for instance, on homepage, in the featured area slider, in archives, widgets and on the post page.
Adding Featured Image to a post.
Go to post settings at WordPress Dashboard > Posts > Your post > Edit. You can add a new post or edit an existing one. Find a box called "Featured Image" on the right side. Select the picture that you wish to add. Its size should be at least 920 x 520 px.
If you’ll turn off the sidebar for a single post, the width of the image should be no less than 1140 px.
If you want the photo to look good on Retina screens, make sure the size is twice as big as the actual display.