Our earth rotates like a spinning top around its own axis. On closer inspection it becomes clear that the angle of the axis to the orbit is not constant, it is influenced by different movements. Precession describes the change in direction of the earth's axis over a period of about 25,000 years.
Precession is caused by torques due to different mass distributions of the earth. It is not a perfect sphere, but rather a flattened ellipsoid. Besides this aspect, the mass attraction of the moon and the sun lead to an straightening of the earth's axis.