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.
This document is designed as being a simple but comprehensive introductory publication for anybody trying to get into the Semantic Web: from beginners through to long time hackers.
Chapters: History, sequential programming, concurrent programming, error handling, advanced topics
They say it takes four days to complete the course. If you know a little Prolog and a little LISP it takes you rather a few hours.