I believe you have already seen a swing. A device that you sit your kids into and push them back and forth. Ok, just to make sure. You probably noticed that the swing goes back and forth at a regular rythm. That's the resonance frequency. The natural frequency of the swing.
Now to make things a little complicated, you can take the swing with both hands and shake at any frequency. But if you rock it at its resonance frequency, you will see that with little effort you can increase it amplitude.