In general, We know how to use of images in post and pages in WordPress. Featured image is another smart way to use an image for post and pages in wordpress. This feature depends on your theme. Your theme should have support this feature. Otherwise you’ll not be able to use this option.
How do you understand that your theme supports this feature or not?
Look at the top right corner and click on “Screen Options”. If you find a check box “Featured Image” there, that implies your theme support this option.
If you select this check box, you’ll find featured image module on the bottom right corner of Add New or Edit Post page on the dashboard.
How to add featured Image?
If you are adding new post/page or editing existing post/page, you’ll find “Set Featured Image” link inside the featured image meta box located right bottom corner or below page attributes meta box. Click this link to open the WordPress Media Uploader. You can use that to upload an image from your computer or use an existing image from your media library. Once you select the image, simply click on Set Featured Image