Marty Eigner: It shows you areas of your website where customers are interacting.
Dave Elliott: Basically, its used for checking user experience and how users are consuming your content.
a heat map will show you where your users are clicking on the page and where they are mouse hovering etc. Some of the better ones will follow users(by creating a video) through out their entire journey on the site, so you can see exactly what route they take, what they click on how long they stay there and other things.
Really useful information but a massive rabbit hole to jump down.