Acceleration
Acceleration is how quickly something speeds up, slows down, or changes direction.
Think of it this way
A ball rolls across a tilted tray. At the top, it moves slowly. As it rolls down, it moves faster and faster. That change in speed is acceleration.
Explaining acceleration by grade level
Roll a ball down a ramp. It starts slow at the top. Watch it move faster and faster as it goes down. The ramp makes the ball speed up more each second.