We're all familiar with the annual march of the seasons, and that the usual transition from summer to winter is due to the tilt of the Earth's axis with respect to our sun - when our orbit carries us so that the Northern Hemisphere is pointed away from the sun, it's time to break out the coats and hats.
If you've ever spun a top, you know that at first it spins perfectly straight up, but as it slows down it starts to wobble. There's an extra rotation, called precession, added to the spinning of the top itself. The Earth is spinning, and it's tilted, so there should be no surprise that it too wobbles, or precesses.
This means that the north star, Polaris, isn't always the north star. It's not exactly fast: our axis of rotation takes about 26,000 years to lazily trace out a circle in the sky. But people have been around long enough to notice. The ancient Egyptians, for example, used Thuban in the constellation Draco to align themselves, and thousands of years from now the bright stars Deneb and Vega will get to take their turn.