The light-year is a way of measuring distance - and not time as you might think from the name.
It is the distance that light travels in one year.
You may not think that light takes any time at all to travel - after all, there is no pause after you turn on a light switch, but that is because it travels very fast. In fact, it moves 300,000 km every second. That is more than 7 times around the Earth in a single second!
The light-year is, therefore, a very long way; it is almost 9,500,000,000,000 km. For comparison it is just 17,000 km from London to Sydney, Australia. So in the vast expanse of the Universe, the kilometre is just too small a measurement and a light-year is much more useful for measuring the huge distances between stars and galaxies. The following are a few examples of typical light-travel times:
Of course, because light takes so long to travel between stars and galaxies, when you look at very distant objects, you are actually looking back in time! Astronomers use this to look at the history of stars and even the whole Universe.