This is an illustration of the classical proof of Pythagoras' theorem.
There are many other proofs, but this is the one from Euclid.

Play around with the vertices, A, B & C.
Can you describe carefully the construction shown here?
Why are the four coloured triangles equal in area (green=yellow=pink=blue)?
Can you find a rectangle having the same area as the square BCIH?
How can you conclude that ?