What explains the precession of the equinoxes?
The Earth's axis of rotation undergoes gravity induced precession which causes the positions of the equinoxes to rotate with respect of the fixed stars.
The Earth's axis of rotation undergoes precession by where is sweeps out a cone with a period of abut 26,000 years. This is also known as the precession of the equinoxes.
The precession is similar to that of a spinning top. It is caused by the gravitational pull of the Sun, Moon and other planets.
This means that the direction of the equinoxes slowly rotates with respect to the positions of fixed stars - very distant stars positions change so slowly that they can be considered fixed.
The direction of the Vernal Equinox is used as one of the coordinate axis for measuring right ascension used by astronomers. The amount of precession needs to be known so that stars' positions can be determined accurately.