When the USGA added slope rating to the pre-existing course rating, in the early 1980s, it did so because of something that had always been obvious. The more difficult a course is, the greater the descrepancy is between the scores of stronger and weaker players.
The course rating system did not account for that factor; slope does.
Imagine two players, Player A and Player B. Player A is a six handicapper, Player B is a 16 handicapper. On a course of average difficulty (slope of 113), both players are likely to play close to their handicaps.
But as course difficulty increases, Player B's scores will rise faster than Player A's. On a course with a slope of 135, Player B might need an additional 10 strokes, while Player A might only need an additional 3.
Now, imagine that descrepancy plotted on a graph. The graph lines of both players A and B would rise as the course difficulty increased - but the line of Player B would rise at a much steeper angle than that of Player A.
The slope of a weaker player's scoring graph is steeper than that of a stronger player's. And there you go. The term "slope" - referring to relative steepness of the graph line of a weaker player's scores - was chosen to represent the USGA's new method of rating course difficulty for bogey golfers.
Hey, they had to call it something.
