A light-year is a measurement of distance, often used in astronomy to express how far light travels across space in a standard Earth year of 365.25 days. Because light travels so fast – 300,000 kilometres (186,000 miles) per second – numbers get too messy when dealing with galaxies and stars beyond the solar system.
One light-year is equal to 9,500,000,000,000 kilometres (about 6 trillion miles). Or, to be more precise, one light-year is 9.460730472580800 trillion kilometres (5.878625 trillion miles).
When considering Proxima Centauri, the nearest star, it’s a lot easier to express the distance as 4.243 light years as opposed to 40,018,889,899,016.78 kilometers. Would you rather say the Andromeda Galaxy is 2.5 million light years away or 23,651,826,181,452,000,000 kilometres away?
Short answer: A light-year is a measurement of the distance light travels through space in a year. It is often used to express distances beyond the solar system.