A light year is not a measure of time. It is a measure of distance. Stars and planets are so far away that if we
measure the distance in kilometres, the number would be enormous. So, these huge distances are measured in 'Light years'. Light travels at a speed of 2,99,792.458 km per second. So, a light year is the distance travelled by light in one year 9,460,000,000,000 km.
Proxima Centauri, our nearest star, next to the Sun is 4.2 light years away from us.