Question: what does a million light years mean?

James Harpur answered on 14 Mar 2017:

A light-year is a unit of distance. It’s about 9 trillion kilometers.

Waves of light travel at a speed of 300,000 kilometers a second (km/s). The distance traveled by a wave of light in one year is what is known as a light year. So a million light years is a very, very, very large distance.

Light years are also interesting because they tell us how far into the past we’re looking at. That does sound weird but when you look into space, you are looking into the past, and this is why.
Let’s say you are looking at the sun, and for some mad reason the sun suddenly turned green. You would say to someone ‘the sun just turned green now’, but actually it turned green 8 minutes ago and you are only seeing it now. What happened was the sun starting giving out green waves of light, these brand new green waves then traveled this massive distance to your eyeball, telling you ‘hey I’m green now’. That distance from the sun to your eye takes 8 minutes for a wave of light. Meaning when you look at the sun, it’s not a live update, it is the image of the sun from 8 minutes ago. We say the sun is 8 light minutes away.

So now extend that to the stars and galaxies, where lots of them are millions and billions of light years away from us. Therefore, if you take a picture of a galaxy that is say, a billion light years away from us, you are looking at it as it was a billion years ago, it may not even be there anymore.