Others have mentioned the "expanding ballon" analogy. But personally I find this more intuitive: Imagine you are baking sweet rolls with raisins in them. When

Others have mentioned the "expanding ballon" analogy. But personally I find this more intuitive: Imagine you are baking sweet rolls with raisins in them. When you are proofing your dough, the yeast makes the dough expand in each point, so all the raisins become further apart. Now imagine starting with an infinite dough. As it is proofing, it will still be infinite, but there will be a measurable increase in the distance between raisins, and the speed of how quickly a specific raisin moves away from another one increases linearly with how far they are apart when you start measuring. That's all "expansion of space" really is. We can see other stars moving away from our own, and the stars far away are moving away faster than those close to us. And since looking outwards is looking backwards in time, we can only see as far away as corresponds to the first stars that were formed. And maybe you've heard that we can observe "expansion is slowing down". That is actually the fact that we measure a deviation from linearity in speed vs. distance.