Acceleration is how much your speed changes in a certain amount of time.
Picture yourself standing in front of your house. When you stand still, your acceleration is zero. When you begin to walk, your speed is changing – that’s what acceleration is – it measures how fast your speed is changing. If you begin by walking slowly, then gradually start to go a little faster, until after some time you are running, that’s a slow acceleration. If you start running right away, going right from standing still to running, that’s a fast acceleration.
People often mix up acceleration and speed, but they’re not the same. Suppose you started out running, and then you kept on running at the same speed. Your acceleration would be zero – just like when you were standing still. You have acceleration only when you speed up or slow down, no matter how fast you are going. (Slowing down gives you negative acceleration, or deceleration).