Pythagoras' Theorem is an incredibly useful bit of mathematics. It applies to every single right-angled triangle you could possibly draw; if you spent the rest of your life drawing right-angled triangles, you would not find one that Pythagoras' Theorem did not work for.
Pythagoras' Theorem is often quoted (or misquoted) using lots of long words. This makes it sound more complicated than it actually is.
Imagine taking a right-angled triangle and drawing a square on each side. The area of the square on the longest side (called the hypotenuse) is the same as the area of the other two squares added together. You can watch a demonstration of this on YouTube.
Rather than write out all the long words, we can simplify Pythagoras' Theorem by using algebra to write a formula. If we label the sides a, b and h, with h being the hypotenuse, or longest side, then the areas of the squares are a², b² and h² respectively. So we can write a² + b² = h² as a much shorter version.