Some people think a light-year is a time period. Maybe it's because of the word "year" that it comes to confusion.
It's simple though, it's the distance light travels in one year. The speed of light being 299 792 458 meters per second, roughly 300 000 km/s, the distance of a light-year is in fact enormous.
The light reaching the Earth from the surface of the sun takes about 8 minutes, that is a distance of about 149 600 000 km. That means that we see the sun as it was 8 minutes ago.
The further the observed objects are, the further we travel back in time. Proxima Centauri, the next star, is about 4,243 light years away. That also means that the light we're seeing now from that star left 4,243 years ago.
The nearest galaxy, the Andromeda galaxy, is 2 538 000 light years away, so we know today what it looked like 2 1/2 million years ago.
In December 1995 the Hubble telescope took a picture of a tiny dark place in space (Hubble Deep Field), and the result was astonishing. Around 3000 dots have been discovered, all galaxies, and some are about 12 billion light years away. The light from those galaxies is therefore 12 billion years old, that's way before the existence of our sun.
The pictures taken from the universe can therefore be compiled as a timeline of the universe.
That also implies that we will never see actual images of our universe, the age of the picture depends of the distance of the object.
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