The universe is expanding.
If you zoom out far enough, you'll find that everywhere in the universe is moving away from everywhere else.
But once you know the universe is expanding, you might wonder if it's going to keep expanding somehow, or eventually stop and crunch in on itself because of the attractive force of gravity.
Well, all you have to do is point your telescope at a distant galaxy, measure how fast it's moving away from us, wait about a million years, and then do the same thing again to see if the galaxy is moving faster or slower than before.
But if you don't have a million years to spare, here's what you do. You look at different galaxies.
Because light takes time to reach us, when we look at a galaxy far, far away, we see it as it was a long time ago.
So looking at a bunch of galaxies different distances away is kind of a rough proxy for looking at the same one at different times.
The light from the stars in a distant galaxy can tell us two basic things.
First, its speed: light from anything moving away from you becomes redder in color (kinda like how police sirens get lower in pitch when moving away).
This red shifting of light tells us exactly how fast stars are moving away from us.
