Achilles was the great Greek hero of the Trojan War; his mother was the sea-goddess Thetis. When it came time for Achilles to go to war, his mother arranged for Vulcan, the craftsman of the gods, to make him a suit of armor and a shield. Notice how the shield shines so brightly that Thetis can see her own image reflected there! The painting is one of the frescoes preserved at Pompei. The shield of Achilles was made famous in a description in Homer's Iliad; you can read more about the shield at Wikipedia.