Testu01 is like a big book of tests that helps you make sure things are working the right way. We use this book to check that computers and other machines are doing things in a fair and random way. Imagine you have a jar filled with red and blue M&M's. You want to make sure that there isn't a secret plan to give you only the red ones. So, you use the tests in the Testu01 book to check that the M&M selection is truly random and not biased towards one color.
The reason why we use Testu01 is because we want to make sure that everything is working as it should be. Imagine you have a robot that is supposed to make pancakes. You want to know that the robot is not making pancakes in the same way every time. Testu01 checks that the robot is making the pancakes randomly, which is what we want it to do.
So, Testu01 is like a very important tool for making sure things are working right. By using it, we can be confident that the things we build and create are working in the way that they are supposed to.