Hello,

light-year is a unit of distance. It can be defined as...It is the distance that light can travel in one year.

..... Light moves at a velocity of about 300,000 km each second.....

See,measuring in light-years also allows astronomers to determine how far back in time they are viewing.Because light takes time to travel to our eyes, everything we view in the night sky has already happened. In other words, when you observe something 1 light-year away, you see it as it appeared exactly one year ago

Hope this helps

Good luck