They violate the Rules of Golf. Whether the ball used for a mulligan must be played is always a subject for heated debate. A handicap is a number that represents your skill as a golfer.
Good players have low handicaps.
When a good player plays a match against a bad player, the good player must give the bad player strokes based on the difference between their handicaps. If you belong to a golf club or association of golfers, your club can compute a handicap for you. Most also have programs for you to form your own club or association which could compute your handicap. There are lots of different types of handicaps. Course Ratings are established by an evaluation of the course by members of a committee, usually from your regional golf association.