Slope is as Jim Hoak indicates. It measures how much difference there is in the difficulty of a course for scratch golfers vs. an 18 handicapper. It is not a measure of difficulty per se.
Most good players would say that Pinehurst #2 is a much tougher course than Tobacco Road, but TR's slope is 150 and #2 I believe is in the 120's. Pinehurst #2 has a relatively low slope rating since bogey golfers can still get it around making their bogies, while low handicap golfers have to play very well to make their pars. On the other hand, Tobacco Road has an exceptionally high slope rating since scratch golfers can get it around there fairly easily as they aren't intimidated by the blind shots, visual trickery, and so on, and can make lots of birdies with the short par-5's and par-4's. But, the typical 18-handicap has a tougher time keeping the ball in play and then recovering from misses.