The Fourier Transform, explained in one sentence
Fourier Transform became trivial to me when I noticed that it's just a basis transform, as you would do with ANY other basis. Except this basis happens to be sine and cos waves of different frequencies.i.e. you view the entire signal in the original domain as a single point in space of dimensionality equal to number of measurements, and then you project it onto the new (fourier) basis. For example, one of the new basis vectors could be sine signal of some frequency k. You find the component of the original signal along this new basis just as you would do for any other basis vector, with a dot product. The only slight complication is that to get the phase information you have to actually dot with a cosine at that frequency as well. The confusing e^ix complex number multiply "hides" the fact that we are actually doing two simultaneous dot products (since e^ix = sinx + icosx): one with the sine and the other with the cos, to get both the frequency and phase.this is the intuition for DFT but for FT you just have infinite-dimensional vector space, done. This is much more intuitive to me than "spinning a signal in circle", which I personally find very to be a very confusing statement.EDIT: granted, basic linear algebra concepts (vectors, basis transform) needed.