Using the Fourier transform, any signal can be analysed into its frequency components.
The diagram shows a recording of speech, and its analysis into frequency components.
With some signals it is easy to see that they are composed of different frequencies: for instance a chord played on the piano is obviously made up of the different pure tones generated by the keys pressed. For other signals the connection to frequency is less obvious: for example a hand clap has a frequency spectrum but it is less easy to see how the individual frequencies are generated.
You can use a piano as an acoustic spectrum analyser to show that a hand clap has a frequency spectrum:
Each string of the piano acts as a finely tuned resonator.