Newton's Second Law of Motion explains the relationship between the force applied to an object, the object's mass, and the resulting acceleration. This relationship is expressed as

F = ma where

F = the sum of all external forces acting on an object
m = the object's mass
a = the object's acceleration

If F = ma, then a = F/m. The acceleration of a car equals the sum of the external forces applied to the car, divided by the car's mass.

If the forces acting on a car balance out, for instance, if the driver applies the brakes and produces friction equal to the force that the engine is applying to the wheels, then F = 0. The car doesn't budge. In this case, acceleration also equals zero, because zero divided by anything equals zero.

Newton's second law makes intuitive sense. For a car of any given mass, the more unopposed force is applied, the faster the car accelerates. Hold the unopposed force constant, and the more massive the car the slower it accelerates.