Many students feel that, since both are measured in kilograms or pounds, both are really the same thing. Mass resists motion. We should exclude here motion without acceleration, which by Newton's first law can continue by itself indefinitely. Mass resists acceleration . Newton guessed the reason.
All material also resists acceleration.
A big stone with 10 times the weight of a small one also has 10 times the resistance, and therefore it does not fall any faster. Newton named the resistance to acceleration intertia . We call it mass . If the only use of the concept of mass was for explaining why big and small stones, in free fall, accelerated at the same rate, it would not be very useful. Weight does not drive such motions, but inertia remains an important factor.