Here is a YouTube video about Explaining a Fourier Series by Drawing Circles, and is part of a growing collection of videos I have found to be useful. The reason is showing how any complex signal or image may be expressed with a Fourier series and understanding science related to software defined radio.
This tutorial, "Explaining a Fourier Series by Drawing Circles" is free education you can watch, which will help you deepen your knowledge and be better prepared for licence exams, schoolwork, or operating radio equipment.