What Is A WordPress / Genesis Hook?
A "hook", when it comes to WordPress and the Genesis Framework, refers to an invisible code snippet that is placed at a particular location on a WordPress website that can "receive" custom code to either display or execute additional content and/or functionality.
So, for example, if you wanted to place a banner ad right after your blog post content so readers would see it after finishing your latest article, you could use some custom code to "hook into" the appropriate Genesis hook and display that banner ad where you want it. An appropriate Genesis hook in this case might be the genesis_after_entry hook, though there are several others that might work just as well.
Note: For a visual reference of the many front-end Genesis/Dynamik Hooks be sure to check out THIS DYNAMIK DEMO SITE.