Bogey golfer, good. Scratch golfer, better. Sub-par golfer, best. Most golfers, statistically, are outside these three categories. A scratch golfer needs no strokes to score the Course Rating on any course. That's basically it.
In its Handicap System Manual the USGA defines a scratch golfer as follows:
A "scratch golfer" is a player who can play to a Course Handicap of zero on any and all rated golf courses. A male scratch golfer, for rating purposes, can hit tee shots an average of 250 yards and can reach a 470-yard hole in two shots at sea level. A female scratch golfer, for rating purposes, can hit tee shots an average of 210 yards and can reach a 400-yard hole in two shots at sea level.
Mathematically, then, when you apply the following formula to a scratch golfer, the resulting Course Handicap would be zero:
Course Handicap = Handicap Index x (Slope Rating/113) = 0
Since the highest Slope Rating is 155 (difficult) and lowest is 55 (easier), mathematically a scratch golfer's Index will fall between +0.3 (think -0.3) and 0.3 in order for the above formula to always produce a zero Course Handicap when you round to whole numbers. When someone has an Index better (lower) than zero, a "+" symbol is used (you have to "add" strokes to arrive at the Course Rating).
A related topic, the USGA's Course Rating is defined using the playing ability of a scratch golfer:
A "USGA Course Rating" is the USGA's mark that indicates the evaluation of the playing difficulty of a course for a scratch golfer under normal course and weather conditions. It is expressed as strokes taken to one decimal place, and is based on yardage and other obstacles to the extent that they affect the scoring ability of a scratch golfer.
If you play in a "net" event which takes your handicap into account, you receive the number of strokes you require to play to the level of a scratch golfer. Said a different way, you receive (or give) the number of strokes required to play to the Course Rating for that course and set of tees.
Seems like a simple enough phrase, but "scratch golfer" can scratch up a whole lot of confusion!