A light-year is a unit of length used to express astronomical distances and is equivalent to the distance that light travels in one Earth year. Light travels at a constant speed of 299,792,458 meters per second in a vacuum. Therefore, a light-year is about 9.46 trillion kilometers (9.46×10^12 km) or 5.88 trillion miles (5.88×10^12 mi). It is important to note that a light-year is a measure of distance and not time, despite its name.

To put this into perspective, the nearest star to our sun, Proxima Centauri, is about 4.2 light-years away. This means that the light we see from the star takes a little over four years to reach us. The sun is about eight light-minutes away from Earth, while the moon is just over a light-second away. Scientists use these terms when talking about communications with deep-space satellites or rovers.

In summary, a light-year is a unit of length used to express astronomical distances and is equivalent to the distance that light travels in one Earth year. It is about 9.46 trillion kilometers or 5.88 trillion miles.