The distance that light travels in a vacuum in one year.
It is about 9.46 trillion kilometers, or 9.46 x 1015 meters
Proxima Centauri, our nearest star (apart from the Sun), is about 4 light years away.
In other words: if we go at the speed of light (about 300,000 km per second) for just over 4 years we would cover the distance to it.
Note that light-year is not a measure of time.