A light year is a measure of distance: it’s the amount of distance that light will take a year to travel across (assuming the light is travelling in a vacuum).
A light year is ridiculously huge. The diameter of the Earth is 1/7 of a light *second*. The distance from our Earth to the Sun is a mere 8 light minutes. Even Pluto is only about 9 light hours away from our Sun.
The nearest star to us, proxima centauri, is 4 light years away. And here’s one for scale: our galaxy has a diameter of 70,000 light years! Pretty big!
…ok, not much to add from me then, except maybe completing Zachary’s list of scales. The visible universe has almost 50 BILLION light years radius.
Now, here’s where many people (including me, until recently 🙂 ) get confused. The universe is less than 14 billion years old, and nothing can travel faster than the speed of light, so how can we see things that are 50 billion light years away? Well, the answer is that space has been expanding while the light flew through it. The objects we are seeing in about 50 billion light years distance were only about 14 billion light years away when the light headed off towards us, and then that point moved further away from us. But from the point of view of the light itself, it has only traveled a distance of 14 billion light years.
Well, the “only” in the previous sentence may sound a bit funny. It’s still a lot more than my old car will ever manage to do. 🙂