It is the number of strokes a player needs to complete a round, compared to the number of strokes stipulated by the course. If you add up the par for each hole you get the score or par for the course. To calculate the handicap we take as a reference the number of strokes in which the course has been played and subtract the par of the course. The resulting number is the player's handicap.
If the total strokes taken by the player coincide with the total number of strokes required by the course, the handicap is said to be "under par". This rating is used to allow golfers of different levels to compete in the same tournament.