All objects that can vibrate have a natural frequency of oscillation. If you hold one end of a ruler on a desk and push down and then suddenly release the other end you will see it vibrate. The natural frequency depends on the material and its width, thickness, and length.

Resonance occurs when an external oscillating force is exerted on an object that can vibrate. When the frequency of the external force equals the natural frequency then the amplitude of the oscillation reaches a maximum. This condition is called resonance. A very small external force is needed to create a large oscillation amplitude.

You can explore resonance with a mass, like a yo-yo or a heavy metal washer on a string. Hold the top of the string still and pull the object to one side and watch it oscillate at its natural frequency. Then shake the top of the string at the same frequency and watch the amplitude of the oscillation increase. You will have found the resonance frequency. If you raise or lower the shaking frequency you’ll find that the amplitude of the oscillation is smaller.