What is “narrative nonfiction”, exactly? Isn't every nonfiction narrative?

Probably the best example I can give you of narrative nonfiction is the work of John McPhee, who has written largely for The New Yorker. To quote from the Wikipedia article about him:

McPhee's subjects, reflecting his personal interests, are highly eclectic. He has written pieces on lifting body development (The Deltoid Pumpkin Seed), the United States Merchant Marine (Looking for a Ship), farmers' markets (Giving Good Weight), freight transportation (Uncommon Carriers), the shifting flow of the Mississippi River (The Control of Nature), geology (in several books), as well as a short book entirely on the subject of oranges. One of his most widely read books, Coming into the Country, is about the Alaskan wilderness.

Like other practitioners of the genre, McPhee tells a story in an engaging, personal way that often injects something of the author's point of view. Often he even becomes part of the story. He once went on a cross-country trip with an over-the-road trucker to document that lifestyle, becoming part of it himself. You might suppose that driving thousands of miles in a big rig must be pretty boring, but McPhee finds ways to make it engaging — turning it into a story full of interesting details, observations, and opinions. For example, did you know that truckers don't use their brakes on downgrades except in emergencies? With the enormous weight they need to slow down, their brakes would wear out very quickly, so they have to resort to other strategies. McPhee tells you this, and also what happens to truckers who don't make it. Always it is a narrative told about someone by the person sitting in the passenger seat right next to him. That is what makes it narrative nonfiction.

Here is the opening paragraph of that book, Uncommon Carriers, taken from Amazon. It should give you some of the flavor of the writing.

I don't know about you, but that makes me want to keep on reading.