Fourier analysis breaks a waveform into its pure sinusoidal frequencies. Here we see a square wave (like the kind used in telephone dialing tones) broken into the three lowest frequency components. There are really an infinite number of components, although the higher frequency ones are of smaller and smaller amplitudes. Another interesting fact about square waves is that they only have odd integer overtones, i.e. they have a fundamental frequency, an overtone at three times the fundamental frequency, one at five times, one at seven times and so on.