There is a nice GIF illustrating a technique called "frustum culling" in this Kotaku article: http://kotaku.com/horizon-zero-dawn-uses-all-sorts-of-clever-tricks-to-lo-1794385026
The interwebs being what they are, this has also led to some controversy.
Some people have interpreted the opening sentence "Every time you move the camera in Horizon Zero Dawn, the game is doing all sorts of under-the-hood calculations, loading and unloading chunks of world to ensure that it all runs properly," as being about the GIF; that's not what frustum culling does, but that's probably not what the article's author meant anyway.
