In the example above we have an object with constant speed.We increase the speed every time we press the first button.The rate of change of speed per unit time is the acceleration of the object.Accelerations are vector quantities (they have magnitude and direction) so the correct definition is that Acceleration is the rate of change of velocity of an object with respect to time.

A Constant acceleration gives us a constant change of Speed.Note that:

if we give the object acceleration even when we eliminate its speed the object stops moving only temporarly.
If we want to stop the movement completly we have to set both speed and acceleration to zero

If the object has constant acceleration if we set acceleration to zero the object mantains its current speed.

TIP! Try experimenting with all the above buttons with different compinations.