Have you seen a video clip of a guy on a trampoline bouncing sky high? He bounces, and there is a ring of guys around him who also jump on the trampoline. They jump at the right time, adding to the bounce of the trampoline. If they jumped at the wrong time it would not work, because it would not be in sync with the central bouncer's bounces. That seems relevant to resonant frequency.
 
 
 
 

The swing is a classic example of resonant frequency. You push a swing at a certain rhythm, you can just think of it. To make the swing go far you do not go at a faster or slower rate, but you reinforce the existing swing.

And there is the fact that the sky is blue, Rayleigh scattering. The science communicators usually don't say this, but it is because of resonant frequency. Atoms get slapped around by light, i.e. electromagnetic waves, at a certain rate based on the colour. The light causes the lighter electrons to vibrate back and forth, while the heavier nucleus sits there. It's like the electrons are attached to the nucleus by springs (opposite charges attract), so they will vibrate at a certain resonant frequency. The vibrating electrons, accelerated charges, lead to new electromagnetic waves in all directions, the blue light. Note that this will not happen so much if the incoming light is not the resonant frequency of the atom, like pushing a kid on a swing at the wrong rate.
 

There is also the classic opera singer breaking a wine glass by singing. If you get everything right and sing the right note you can break the glass by adding to the existing vibrations in the glass. How do you know the right note, the resonant frequency of the wine glass? By flicking the glass. Ding!
 
 
 

There is a point to this. I will give an amateur science lesson to a young lady who is a hardworking budding musician and sound engineer. For the traditional test at the end she will be asked to calculate the resonant frequency of a room. Like her, you can poke around figuring out the way to calculate this. But the formula is going to be resonant frequency = speed of sound / length of room. Alternatively you could make noises like an orangutang.

* Some excitable, speculative people have remarked about the acoustics in the Great Pyramid of Giza, and elsewhere. I don't know what's going on there. But rooms .. do that. It helps that it is uncluttered.