Resonance

If I hold one end of a slinky and let the other oscillate freely we have a free vibration system. If I move my hand up and down I force the slinky to vibrate. The frequency of my hand is the driving frequency.

When the driving frequency is lower than the natural frequency the oscillations have a low amplitude

When the driving frequency is the same as the natural frequency the amplitude increases massively, maybe even exponentially.

When the driving frequency is higher than the natural frequency the amplitude of the oscillations decreases again.