What's Fourier Series???

Who invented fourier series? I just feel like strangling that person (if the person's not dead yet)...it's freaking hard!!! Prager oh Prager...stop talking at the speed of lightning...i can't understand you!!! This applies to Udrea as well...I dont want lectures to end 20 min earlier! I WANT TO UNDERSTAND YOU! ARGH!!!!!!!!!!!!!!!!!!!!!