Caret Browsing is when you navigate around a page by using the keyboard arrows.
Think of how you use them in a text box, but across the whole page.
This is different to spatial navigation, as spatial navigation only moves across inputs and links.
For those who only want to use a keyboard (or indeed those who can only use a keyboard), being able to navigate webpages in this fashion could be preferable to spatial navigation or the arrow keys which force you to move whole pages at a time.
The same navigation methods as text editors also work. For example, in caret browsing you can also use ctrl+shift+arrow keys to highlight and select text, without needing to use the mouse.
What might this kind of navigation look like?
Internet explorer and Firefox both support caret browsing by pressing "F7". There are extensions that introduce some support for chrome based browsers.