Force equals mass times acceleration. This is Newton's second law of motion, and it describes the way in which objects respond to forces that act upon them.
Newton's first law states that an object that is acted upon by equal opposing forces remains at rest until the forces become asymmetrical. When that happens, the object begins to accelerate in the direction of the greater force. Newton's second law of motion, F = ma, describes this acceleration. The acceleration of the object is inversely proportional to its mass. Put another way, the more massive the object, the greater the force required to accelerate it.