Newton's Second Law states that the amount an object speeds up or slows down, or accelerates, depends on the force of the push or pull divided by the mass of the object.
That means that if you push a heavier object with the same amount of force that you use for a lighter object, the heavier object won't accelerate as much. Makes sense, right?
An object is accelerating if its velocity changes. Remember velocity is a vector quantity so an object's velocity changes if there is a change in speed or direction.