Okay kiddo, have you ever played with blocks and noticed that some blocks look bigger even if they have the same length as other blocks? This happens because the way we hear sounds also has to do with the length of things!
Pythagorean tuning is a way of making music by using a special ratio between the lengths of the strings or pipes that make the different sounds. When you pluck a guitar string, for example, it vibrates back and forth to make a sound. But if you make the string half as long, it will also make a sound, but it will be higher.
Now, imagine you have a longer string and a shorter string that are twice as long or half as long as each other. If you play them together, they will make a nice sound because the vibrations will match up perfectly. This is what Pythagorean tuning is all about!
Musicians use this idea to make sure that the notes they play sound good together. If they use this special ratio between the lengths of the strings or pipes, the notes will harmonize and sound pleasing to our ears. That's why we love music so much!