The slope rating of a course is an indication of the level of difficulty of playing that particular course in relation to a standard USGA golf course.
The higher the number, the more difficult it is to play. Slope rating is used in conjunction with course rating when calculating a handicap index.
Course rating is an indication of the score you would expect to obtain on a standard golf course, whereas a slope rating reflects how much more difficult or easier it is compared to a standard golf course.
Your handicap index is determined by taking the average of your scores from different courses and then subtracting the course ratings from each one which will give you your handicap index for those courses.