One definition of mass is that the mass is the measure of inertia. Inertia is the tendency of the objects to resist the change in motion, more specifically, the change in their velocity. This is typically illustrated by the fact that when a car suddenly slows down, the riders in the car fall forward, because they tend to move forward with the original speed.
The first Newton's Law states that an object will remain moving with the same velocity unless there is an external force exerted on the object. If there is the external force, it will be proportional to the mass of the object. The more mass, or inertia, the object has, the more force is needed to change the velocity by a given amount.
Another way to define mass is as the measure of gravitational attraction. The two object with masses m and M will attract each other with the force of gravity
`F=G (mM)/r^2` , which is proportional to the masses of both objects. The more the mass of an object, the more gravitational attraction it experiences.
It has been measured, with great accuracy, that "inertial" mass is equal to "gravitational" one.The Einstein's theory of relativity states that the two masses are in fact the same thing.