I find all these Fourier explanations bad. The basic idea is very simple: Fourier takes a function, and converts it into the reciprocal domain. So if your X a

I find all these Fourier explanations bad. The basic idea is very simple: Fourier takes a function, and converts it into the reciprocal domain. So if your X axis is time, t. Then Fourier gives you 1/t. What’s 1/t if t was some unit time? Frequency. If you want a nice intuitive example of this, (hand waving begins) a lens will give you a Fourier transform on its back focal plane if all the light coming in are parallel. Don’t quote me on that I need to double check the precise conditions. But you can send laser light through an image printed on transparency and manually apply filters to clean up small artefacts.