Imagine you have a bunch of little blocks, like Legos or wooden building blocks. Now, pretend each of these blocks has a special power to help it do different things. Some might be really good at sorting things, others might be really good at counting things, and some might be really good at finding mistakes in computer programs.
These special blocks are what we call FOSD program cubes. FOSD stands for "Foundations of Software Design," which is just a fancy way of saying the things that help make computer programs work properly.
Now, imagine we take all of these little blocks and put them together to create a big cube. This big cube is like a computer program that has all of the special powers of the little blocks inside it. When we use this big cube, it can help us make sure our computer programs are working correctly and don't have any mistakes.
So, FOSD program cubes are like little blocks with special powers that we can put together to create big cubes that help us make sure our computer programs are working the way they should be.