Any complex waveform can be made up of simple sinusoids.
A square wave is made by first starting with a sine wave then adding every other nth harmonic at frequency nth * the base frequency and amplitude of 1/n
FFT breaks down a waveform into its sinusoidal components to measure the amplitude of the frequencies.
1. Additive sine waves combine to form a complex square wave. 2. The FFT graphs the harmonic frequencies as the square wave takes form.