What idea of aristotle did galileo discredit with his experiments on balls rolling on a incline?

He discredited Aristotle's idea that heavy objects fall proportionally faster than lighter objects. He did it by proving that force was not necessary for motion in his experiment called the Leaning Tower experiment. When objects are moving horizontally instead of vertically, gravity is perpendicular so it doesn't make heavier objects move faster.

Galileo used his inclined plane to examine Aristotelian ideas about motion. Galileo's inclined plane experiment changed these ideas by concentrating on acceleration, a stage of motion which was ignored by Aristotle and most of his followers.