A light-year is the distance light travels in one year. It is not a measure of time, though the name confuses almost everyone at first. Light moves at about 300,000 kilometers per second, so in a year it covers about 9.46 trillion kilometers. That is roughly 6 trillion miles.

The unit exists because astronomical distances are too large for kilometers. The nearest star, Proxima Centauri, is 4.24 light-years away. The center of the Milky Way is about 26,000 light-years away. The Andromeda Galaxy is 2.5 million light-years away. The observable universe is about 93 billion light-years across.

Light-years also measure lookback time. When you see Proxima Centauri, you are seeing it as it was 4.24 years ago. When you see Andromeda, you are seeing it as it was 2.5 million years ago, before humans existed. The farther you look, the further back in time you see.

Astronomers use other units too. The parsec is about 3.26 light-years, and it is the preferred unit for professional work. The astronomical unit is used within the solar system. But the light-year remains the most familiar unit for the public.