From the text: Fourier series is a technique for representing any periodic (or repeating) function across the interval $(-π,π)$ as an infinite trigonometric series. For example, consider the so-called “square wave" function alongside its Fourier series. By adding on an additional term each time, we can see how its Fourier series starts to become a very good approximation for the square wave function. Four diagrams show the development of the Fourier series with an additional term in the series being evaluated each time.