The Pythagorean Theorem states that for any right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. It provides a simple equation, A^2 + B^2 = C^2, that can be used to find the length of any side of a right triangle when the other two sides are known. The document provides an example of using the Pythagorean Theorem to solve for the missing side of a right triangle.