Comoving and proper distances

Comoving distance is like measuring how far apart two things are when they're sitting still and not moving around. Imagine you're sitting in a car and you're looking at a nearby building. Even if you drive away from that building, its comoving distance doesn't change because it's staying in one spot relative to the rest of the universe.

Proper distance, on the other hand, is like measuring how far apart two things are when you take into account their movement through space. Imagine two cars driving down the road. They might be close to each other at one moment, but if they're moving in opposite directions, their proper distance will get larger and larger over time.

To put it simply, comoving distance doesn't change, while proper distance can change depending on how things are moving relative to each other.