It is the hypothetical score that the average scratch golfer would shoot on the course from a particular tee.
Theoretically, an ideal par-72 course would have a Course Rating of 72 from the blue tees. That would mean that a scratch golfer playing a good round would shoot par.
If a par-72 course has a Course Rating of 75, it is very difficult - even for very fine players.
If it has a Course Rating of 68, it is very easy for good players.

A Course Rating is calculated upon the playing styles and abilities of very good golfers. It can provide some insight for mid-handicappers, but it is almost completely irrelevant for high-handicappers
Keep in mind that a 150 meter (airtime) carry over Water can be a heavy challenge for higher-handicapped players

This number exists to tell average players (bogey golfers) what degree of difficulty they can expect to find on a particular golf course.
The actual “slope” referred to is the slope of a line drawn from the low score that a scratch golfer would score on a particular course upwards through the scores of 10, 20 and 30-handicappers for the same course. Hence, the tougher the course, the steeper the slope of the line would be.

If a course has four sets of tees, four different slopes are given. A very short, executive course might have the minimum Slope Rating of 55. As a rule of thumb, any course under 115 is generally considered too easy for players possessing any genuine skills whatsoever. Any moderate to full-length course with a Slope Rating under 100 is too easy to be considered serious golf. The average golfer will be most comfortably challenged when playing from the white tees in the 118 to 128 range. Over that, his game will have demands placed on it that he might not be able to answer. A course with a Slope Rating over 130 is tough. Over 140, it is very, very tough.