Precession is the change in the orientation of the rotational axis of any rotating body. As we all know Earth has an axial tilt of 23.4 degrees to its orbiting plane. For Earth this orientation changes. Not the magnitude of the axial tilt. This phenomenon is known as Precession.
Earth's axial precession is also called with other names: Precession of the equinoxes, lunisolar precession, precession of the equator. For the orientation axis(not the axial tilt - 23.4 degrees) to make a complete revolution it takes 26000 years. In other words, the orientation axis changes its direction by 1 degree every 72 years.
Hipparchus who is known as the founder of trigonometry has first discovered this Phenomenon of Axial precession of Earth. The reason why Earth undergoes precession is because of torque exerted by the Sun, Moon and even Jupiter on Earth.
Because of the Axial precession of Earth, vernal equinox moves through the constellations of the Zodiac as the axial tilt undergoes a cycle/ellipse of orientation change over 26000 years.
The time it takes for Sun to return to the same position along the ecliptic is called a Tropical Year. Since vernal equinox moves along the ecliptic a tropical year is shorter than a sidereal year which is measured as the time taken by the Earth to orbit the Sun once with respect to the fixed stars.
Hence we have a mean tropical year which is an average over all points on the ecliptic and has a length of about 365.242181 solar days.