Terminal Velocity
Terminal velocity is the fastest speed a falling object can reach when air slows it down enough to stop it speeding up.
Think of it this way
When you drop a marble into a jar of honey, it falls fast at first. Then the thick honey pushes back harder and harder as the marble speeds up. Soon the honey pushes up just as hard as gravity pulls down. The marble stops speeding up and sinks at one steady speed — that is terminal velocity.
Explaining terminal velocity by grade level
Drop a coin and a feather at the same time. The coin hits the ground first. Air pushes back on things as they fall. The feather is light and flat, so air slows it down more than the coin.