Is Duo mispronouncing "petryal" here? The audio makes the "y" sound like "uh" but on Gweiadur it sounds like "ee" as if it were a final syllable.
As an adjective, petryal means square as in square/right-angled. As a noun, it means a rectangle or any figure with square/right angles
A square (sgwâr) is just a special case of rectangle.