A light-year is a unit of distance, not time. The International Astronomical Union defines it as the distance light travels in a vacuum in one Julian year (365.25 days), which is exactly 9,460,730,472,580.8 kilometers, or about 9.46 trillion km (5.88 trillion miles).
Light-years are commonly used to describe distances between stars and galaxies. Professional astronomers more often use the parsec, which equals about 3.26 light-years.
Source: Light-year, Wikipedia