A light-year is a measurement of distance, not time. A light-year is the distance a beam of light travels in a single Earth year, which equates to approximately 6 trillion miles. On the scale of the universe, measuring distances in miles or kilometers is cumbersome, given the exceedingly large numbers being discussed. It is much simpler for astronomers to measure the distances of stars from us in the time it takes for light to travel that expanse. For example, the nearest star to our sun, Proxima Centauri, is 4.2 light-years away, meaning the light we see from the star takes a little over four years to reach us.
