In the context used by Graydon, "optical grey," means what appears to be a grey, made by a thin, semitransparent/semiopaque layer of white or other light-value paint over black or other (relatively) darker-value passage. "Optical" refers to what is seen by the eye. In this case what the eyes perceives is grey, though the top layer of paint is white, in the most simplified explanation.
If, as you say, you're a rookie, this isn't really something you'll need to know any time soon. It's a more advanced thing best left alone until you're ready to make good use of it. The most important things in learning to paint well are not the most advanced techniques, but rather, mastery of the more basic ones. First things first. Someone aspiring to become a mathematical will not get there by reading about advanced calculus or trigonometry before learning to add, subtract, multiply, divide, etc., first. The lessons must be taken in order, for them to make sense, in art as in anything. Too many people concern themselves with advanced painting techniques before they have drawing down, and that's always a mistake.
Virgil
