A musical chord is a sound, but it is also a combination of other sounds (the musical notes that compose the chord). Actually, any sound is a combination of other elementary sounds, which are called pure tones. They are just like musical notes, each which its frequency. The spectrogram allows you to see all the frequencies that combine to produce a sound. To try it out, make sure you allow the website to use the microphone. Then speak into the microphone and see what frequencies are present in your voice. You can also use this spectrogram to tune musical instruments or to create music; for example, you can explore different musical scales, and add effects often used by musicians and engineers. This video shows how you can make amazing music with it.
The spectrogram is one of the most commonly used tools in physical sciences and engineering; it is part of the technology behind voice recognition and phone communications.
Note: This tool works well on computers, and has limited functionality on iPads.