Handicap Index is not an average of a golfer's scores, but rather an estimate of what the golfer might shoot on his or her best days.
Handicap Index is figured through a complicated formula that, thankfully, individual golfers are unlikely to ever use themselves. Golfers who carry a USGA Handicap Index usually do so through their local golf clubs, and those clubs keep track of the numbers.
For a lengthier definition of Handicap Index and how handicaps are used, read this overview of the USGA Handicap System or check out our Handicap FAQ.
Handicap Index is used to figure course handicap, which tells golfers how many strokes they can take, and on which holes, during a round.

