The document summarizes the Pythagorean theorem, which states that in a right triangle, the sum of the squares of the two sides forming the right angle equals the square of the hypotenuse. It provides an example of using the theorem to solve the ladder problem, where given the length of a ladder and its distance from a house, it can calculate the height of the window the ladder reaches. The document explains right triangles have three sides - the hypotenuse and two legs - and provides the formula a2 + b2 = c2, where a and b are the legs and c is the hypotenuse.