That means the North Star is mostly visible from the Northern Hemisphere. If you go South, from your perspective, the star appears lower to the horizon. Go further South, and you can't see the North Star at all. He's saying that the Greeks already knew the Earth was round thousands of years ago.

We call the Pole Star the "Pole Star" because it happens to sit directly over the North Pole. Over the centuries the sky shifts and the pole points to different stars; in our century it just so happens that the star we call Polaris sits almost exactly over the North Pole, where usually it or another star is just in the general vicinity. Since the Earth rotates around the pole, the Pole Star never moves in the night sky: all the other stars appear to circle around it over the course of a night.

So if the North Pole was an actual pole that stuck out of the Earth and kept going forever, Polaris is where you would see that pole. If you were standing at the North Pole itself, Polaris would be directly over your head, at the top of the sky. If you were at the equator, it would be right at the northern horizon and you might not be able to see it at all. If you were south of the equator it would be below the horizon, but the southern pole star would now be in view (at the moment there is no bright southern pole star, so we use the constellation Crux which points to it).

And if you are somewhere in between the North Pole and the equator, as is the case with North America, Europe, and Asia, then the Pole Star would be somewhere in between the horizon and the top of the sky. It in fact represents your latitude on Earth; I live at exactly 45 degrees N latitude so Polaris sits at 45 degrees up in the sky. Wherever that star is, that's where North is.

The Pole Star lies exactly overhead if seen from the North Pole. The farther south you travel, the lower it gets in the sky. It's not visible from the Southern Hemisphere at all, due to bulge in the middle portion of Earth. As Hawking notes, in Ancient Greece this was one of the proofs cited for the curvature of Earth.

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