Chromatic aberration (Chromatic distortion or achromatism) is where you bounce light off a mirror or shine it through a lens or other transparent medium and the light isn't focussed on the same point.
The best example of this is shining white light through a prism and getting a rainbow.
It's what makes diamonds throw amazing sparkles.
I used it in a previous picture of a disco ball and since no one commented on it I thought I'd do a tutorial to show that I am far smarter and wiser than you.
TREMBLE MORTAL!