~~You know the counting numbers? 1, 2, 3, so on?
There's infinitely many.
But there's fewer of them than the "reals", which are numbers like 1.5, 42.7, and pi (basically all the whole numbers and all the ones in between).
This diagram is showing the "Continuum Hypothesis" which says that there might be some set of numbers that are "in between" the counting numbers and the reals in terms of size. We (humanity) do not yet know for certain if it's true or not.
This is obviously a gross oversimplification that probably gets some important details wrong.~~
Thank you for the far more detailed (and correct!) explanation.