The document discusses the Pythagorean theorem, which 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 examples of using the theorem to find missing sides of right triangles and to classify triangles as right, obtuse, or acute based on side lengths. It also introduces Pythagorean triples, which are sets of integers that satisfy the Pythagorean theorem.