I'm having a hard time grasping exactly what a Fourier series is. I know the book definition and that it represents any periodic function as an infinite series. I also know that the a0 term is the average value of the function over one period. I can calculate the terms and everything but I don't really understand what I'm doing; I just know how to go through the calculations. Can someone give me a good, conceptual explanation of what a Fourier series is and how it works? Thanks.