what does FFT mag (a.u) means ?

[Jens Noriʇzsɔɥ]

I have seen that you also commented on the final video of our spectra series. Have you watched the second part that goes a bit into detail on Fourier series (Fourier transformations could be understood as a generalisation for non-periodic functions)?

Textbooks on oscillations should typically cover Fourier series and transformations. I personally prefer physics over math introductions in this topic as they are more “vivid”. For a start, Wikipedia should do, too.

… or have I missed the point where you need clarification?