A square is a quadrilateral with vertical and equal adjacent sides.

The title says "two" squares with side A and side B, that is, "a square with side A and a square with side B"

Figures (1) and (2) show two orange rectangles with a length of a and a width of b and a yellow square with a side length of (b-a), which can be changed into a big square with a side length of c by moving the position. Where c is the hypotenuse of a right triangle with right angles a and b.

So 2ab+(b-a)? =2ab+b？ -2ab+a？ =a？ +b？ =c？ ,

In other words, Pythagorean theorem has been proved.