Acceleration is simply how much something changes speed over time. For example, a sports cars might accelerate from 0 to 60 kilometers per hour in 5 seconds. Meanwhile a minivan might only accelerate from 0 to 20 kilometers per hour in 5 seconds. When you ride in a car, you can feel the card speed up and slow down, but you don't feel anything if the car stays the same speed. The accelerometer in your computer only senses changes in just like you.
Perhaps a little more accurate analogy is a boat. The laptop only senses certain kinds of acceleration: the 1) up/down, 2) front/back and 3) side to side motions of a boat.