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, even if you don't want the featured image to display at the top of your post, you should still assign a featured image. Once that's done, you can hide the featured image from the top of your post by navigating to Appearance > Customize > Post Settings > And checking the box next to "Hide Post Featured Image".