A Pythagorean theorem question is one that deals with the side lengths of a RIGHT triangle.
When solving a question you would use the formula, a^2 + b^2=c^2
A and B are both of the sides that aren’t the hypotenuse.
C is distinguishable from a and b as it is the side opposite from the 90 degree angle. (90 degree angles are formed by perpendicular lines)
Hope this helps :)