The mass of an object measures the object's inertia. Inertia is the object's resistance to a change in motion. The greater the mass of an object, the greater will be that object's resistance to moving.
Sir Isaac Newton's "Mathematical Principles of Natural Philosophy" includes three fundamental laws of motion, the first of which is often referred to as the Law of Inertia. This law states that object at rest remains at rest, and an object in motion remains in motion with continuous velocity, unless influenced by an outside force. The amount of force necessary to sway an object from its inertia varies in accordance with the objects mass.