Acceleration is the name we give to any process where there is a change in velocity (velocity is a speed and a direction). There are only two ways for you to accelerate: change your speed or change your direction — or change both. When you press down on the ‘accelerator pedal in a car, the car surges forward, going faster and faster. This change in velocity is acceleration.
When you hit the brake peddle in a car, you go slower and slower, and this is called deceleration.
Watch this video: What is Acceleration? ( Physics in simple terms )