The course rating is the estimated average score of a "model" scatch golfer--[For Men] That golfer carries his drive 225 yards and with roll his average drive is 250 yards--He can hit his second shot 200 yards with a roll of 20 yards--He has the skill of the average player who makes the match play round at the US Am(32nd qualifier)-- His accuracy and recovery skills are based on that "model"-- A course is rated on a standard of obstacales in the driving area and target area(green) and an objective formula taking into account many factors(wind, roll, forced carry, trees,water, bunkers, green speed and severity, etc) which are used to fix the course rating--(ie--73.5)
(BTW the USGA checks their figures each year, and the length of drive may be changed in the future based on the average driving distance at recent US AMs)--
Then a rating is also given for a "model" bogie golfer--He drives the ball 180 with a roll of 20 for a 200 yard drive and hits his second and maximum of 150 with a 20 roll, or 170-- The bogie golfer has less accuracy than the scatch and tables are used to compute his average score on the same course--
The slope is then the relative difficulty of the course for a bogie golfer, vs a scratch golfer, with 118 reflecting the average(the bogie will on the average score approximately 21 strokes higher than the scratch)-- If the slope is higher(say 135) it reflects that the score of the bogie golfer is expected to be more than 21, rather approximatly 24 strokes higher than the scratch-- The higher the slope the greater the predicted difference in score between the scratch and the bogie--
A high slope DOES NOT MEAN a course is more difficult(It would be possible to have a very difficult course with a high course rating--say 78.5, but a very low slope(say 101)--That would mean it would be difficult for everyone, but because of design(a lot of trouble in the scratch driving area, forcing the scratch to lay up, wide areas for the bogie golfer etc) the difference in score between the scratch and the bogie is about 19 strokes, hence a very low slope--The reverse is also true--
The ratings are assigned by USGA certified rating teams, using charts and objective standards to create unifority in ratings. Courses are to be rerated between 5 and 7 years(or when major changes are made)