An Astronomical Unit (AU) is a standard measurement used in astronomy to describe distances within our solar system. It is defined as the average distance from the Earth to the Sun, which is about 93 million miles or 150 million kilometers. This unit helps astronomers easily compare distances between various celestial bodies, such as planets and moons. For example, when we say that Mars is about 1.5 AU from the Sun, it means Mars is 1.5 times farther from the Sun than Earth is. Using AUs simplifies complex calculations and provides a clearer understanding of the vast distances in space, making it easier to study our solar system.