WordPress “Featured Image” is the image that is used as a thumbnail to represent the post in archive listings, on the front page, and (sometimes) at the top of the post itself. Featured Image is not implemented in all themes, but has become very popular.
If your theme supports Featured Images, you will see this meta box on each post edit page.
Once you’ve chosen a featured image, the meta box will look like this.
A few things to know about Featured Images
1) When you select a Featured Image you do NOT choose a specific size, you just select (or upload) a Media Library image. The size used on archive pages or your home page is determined by your theme. Cropping and/or scaling may occur.
2) Different themes implement Featured Images on posts differently. Some themes will add the Featured Image to the top of the post, others will not.
3) If your theme does not support Featured Image, you can add it with some code (ask your developer) or switch to a theme that supports Featured Images such as Genesis. If you switch to a theme that handles thumbnails (Featured Images) differently than your existing theme, you will need to generate new image sizes for everything in your Media Library. You can do this with the Regenerate Thumbnails plugin.