A resonant frequency can be many things. One application of resonant frequencies can be the amplification of sound through a tube of the correct length. Yet it can also be much more physical
Consider a swing. There is a frequency of pushing a person on the swing that will cause them to rise higher, and there are frequencies that will end up accomplishing very little. Any of us who have pushed a swing know that you must push to add momentum, not subtract. The fall of the Tacoma Narrows Bridge is an interesting example of this.