AU stands for astronomical unit, and equals to the distance between the Earth and the Sun, i.e. about 150 millions kilometers (precisely 149,597,870,700 km) or 93 millions miles.

This unit is useful to measure distances in the solar system. For instance, the distance from Jupiter to the Sun is about 5 to 5.5 AU, so more than 5 times further away from the Sun that the Earth.