Fourier theory is pretty complicated mathematically. But there are some beautifully simple holistic concepts behind Fourier theory which are relatively easy to explain intuitively. There are other sites on the web that can give you the mathematical formulation of the Fourier transform. I will present only the basic intuitive insights here, as applied to spatial imagery.
J. Berner, P. Grohs, G. Kutyniok, and P. Petersen. (2021)cite arxiv:2105.04026Comment: This review paper will appear as a book chapter in the book "Theory of Deep Learning" by Cambridge University Press.
T. Nachlieli, Y. Mor, E. Gil, and Y. Kashtan. Eleventh Congress of the European Society for Research in Mathematics Education, 22, Utrecht, Freudenthal Group; Freudenthal Institute; ERME, (2019)
R. Movassagh. (2019)cite arxiv:1909.06210Comment: 18 pages + Acknowledgements and references. 3 Figures. arXiv admin note: text overlap with arXiv:1810.04681.