Okay kiddo, the Ebbinghaus illusion is when you see two circles that are actually the same size, but you think one is bigger because of the sizes of the circles around them.
Imagine you have two pizza slices, and one slice has small pepperonis around it and the other has bigger pepperonis around it. Even though both slices are the same size, the one with the bigger pepperonis will look bigger because of the pepperonis around it.
This is what the Ebbinghaus illusion is like. Your eyes get tricked, and you might think one thing is bigger or smaller than it actually is because of the things around it. It's kind of like an optical illusion.