There is the analogy of a balloon’s surface where every point moves away from its neighbor, or a better analogy of bread expanding as it is baked, since that’s more three dimensions. The idea is that space is expanding at the atomic level at a certain rate, but it’s so small that it takes an astronomical amount of these atomic increases to be able to measure it (we can’t measure expansion at solar system scales, or even between our galaxy’s stars, as gravity drowns out the effect. But space is so large that over distances like between galaxies, the light that has traveled all that way has had to travel over this expanding so much that we can see a shift in its wavelength. And overall everything is shifting red, so either we in our section of the galaxy are the center, or it’s something that’s common in any part of our universe. One of these is far more likely.