a is one length, usually the shortest, b is the side, and c is the hypotenuse(diagonal, longest side of the triangle). Let's say that you take a rectangle with sides 3 and 4. 3² and 4² is 25 which is 5², which is 25. If you try measuring the hypotenuse with a ruler, you'll see that it really is 5. Just keep the equation in mind.