Newton's second law of motion says that the acceleration of an object is dependent upon two variables: the net force acting on the object and the mass of the object.
The greater the force, the greater the acceleration, and vice-versa. If you double the force, then you double the amount of acceleration.
At the same time, the more mass something has, the harder it is to accelerate. To accelerate a 4kg and a 2kg mass by the same amount, I have to apply twice as much force to the 4kg mass.
The second law is expressed mathematically as a=F/m, where a is acceleration, F is net force, and m is mass. It's more commonly rewritten as F=ma (which means the same thing). To get F=ma you multiply both sides of a=F/m by 'm' (remember: when rearranging equations, you always have to do the same thing to both sides).