Explanation:
it shows that all the small triangles that together make the complete square (c²) under the Hypotenuse consist of exactly all the composing triangles of the squares above the legs (a² and b²).
we find the 2 purple, the 2 orange, the 2 yellow and the 2 blue triangles there.
so, the area of c² must be the same as the sum of the areas of a² and b².