This is the “description” for the tag you’re currently viewing. It’s a built-in feature of WordPress that allows you provide some additional information. Maybe even throw in a link here and there. It’s up to you.
This area utilizes the excerpt field on the post edit screen. It’s separate from the rest of the content, so you can write a nice little intro or a brief summary of the post. Maybe even throw in a link here and there. It’s up to you.