Because the earth is at a tilt when we orbit the sun some parts of the earth become in a sense farther and closer to the sun. The earth's tilt is always in the same direction, so it's not really tilting back and forth. If you have a tilted globe at home (or maybe at your town's library) you can use an object in place of the sun and move the globe in a circular motion around it. And it might become a little more clear.
