(Bogey course rating - USGA course rating) x 5.381 = Slope rating
Which means that a slope of 130, for example, on a 72 course rating signifies a "bogey course rating" (what a bogey golfer would normally shoot) of 96. So a course that would be average difficulty for a scratch golfer would be above average difficulty for a bogey golfer.
So the slope is the relative difficulty for a bogey golfer
compared to the difficulty for a scratch golfer. The higher the slope, the greater the difference in score will likely be between the scratch golfer and the bogey golfer.
Augusta National unofficially had a course rating of 76.2 and a slope rating of 148 in 1990 (I'd imagine both are higher these days). That means that the average bogey golfer would be expected to shoot a 103.7 there, nearly 14 strokes higher than his average round. Again, that would probably be more like 106-108 today with the added length, rough, and trees.
By the way, that answers a lot of questions from
What would you shoot at Augusta National