Okay, so let's imagine that you have a big box of colored blocks, like Legos or Mega Bloks. You start building with them, putting one color on top of the other until you've built a tower. But what if you wanted to build a pattern, like stripes or checkerboard? You might start putting the blocks in a specific order - red, blue, red, blue, red, blue...
Now imagine that you're building something really big, like a skyscraper. You can't just keep stacking blocks in the same pattern over and over again, because eventually you'll run out of colors, or the pattern will get boring. So instead, you might use different patterns - maybe red, blue, green, red, blue, green, or red, blue, purple, blue, red, blue, purple, blue...
Fokker periodicity blocks are a way of using patterns to create really complicated structures, kind of like a very elaborate Lego building. But instead of just using different colors or shapes, these patterns are based on something called "group theory". That's a fancy way of saying that we're looking at how different symmetries and transformations can combine together.
Here's an example: imagine a pattern that goes ABABABA... forever. That is, you have an "A" block, then a "B" block, then another "A" block, then another "B" block, and so on. This is an example of a Fokker periodicity block, because it uses a repeating pattern to create something bigger.
But what's really cool is that you can use different patterns, or different combinations of patterns, to create even more complicated structures. Sometimes these structures have symmetries or repeating shapes that you might not expect - just like how a snowflake has six-fold symmetry, or a honeycomb is made of hexagons.
So to sum up: Fokker periodicity blocks are like Legos, but instead of just using different colors or shapes, we use patterns based on "group theory" to create very complicated structures. And these structures can have all sorts of interesting symmetries or repeating shapes!