Resonance

Resonance is the tendency of an object to vibrate with much larger motion when it is pushed at one of its natural frequencies. At that frequency each push adds to the motion left over from the last one, so the amplitude builds up. How far it builds depends on how much energy the object loses to damping on each cycle.

Example: Pushing a child on a swing at the right moment in each swing makes them go higher with little effort, while pushing at random times does almost nothing.