yes, it is the distance traveled by light per year of period (300,000km/s*365day*24hr/day*60min/hr*60s/min). It is just a unit. However, what we perceive by our eyes is also light wave. This is why if you see a star 4 light year away from you and all a sudden explodes, it means it actually explodes 4 years ago and takes the 4 years to be perceived by your eyes...and this is the fundamental concept they combine time and distance for time traveling