The tilt of Earth's axis changes over time. Our current tilt of 23.5 degrees is what gives us our varied seasons - if the Earth were oriented straight up and down with respect to the Sun, each spot on Earth would get the exact same amount of light every day, and there would be no seasonal changes.
The Earth spins, sort of like a top. We're always tilted at 23.5 degrees from vertical, but the direction that the axis points varies, as we can see in the above illustration. (In the center image, the Earth is not really straight. Rather, it is tilted at 23.5 degrees directly out of the page, towards you, and so it looks like it's straight up and down!) Eventually, we arrive at the directly opposite orientation as we have now:
The reason that our "north star" changes over time (as well as the reason that our zodiac symbols, were we being truly accurate, are not now associated with the correct months of the year) is because where our "north" is, in relation to the stars, is actually changing!
