A handicap is a number representing how many strokes above par a golfer usually plays, letting players of different standards compete fairly. Under the World Handicap System it is calculated from the best eight of a player's last twenty rounds, then adjusted for the difficulty of the course being played.
Using the best eight rather than the average is deliberate. A handicap is meant to describe what a player is capable of on a good day rather than what they average, which stops it drifting upwards after a run of poor rounds and makes it harder to manipulate.
The course adjustment is what makes handicaps portable. Two courses of the same par can be very different to play, so each is rated for difficulty, and a player's handicap index is converted into a course handicap for the specific course and tees they are using that day.
The system was unified in 2020, bringing six regional systems used around the world into one, so a handicap earned in one country now travels to another. Professionals do not use handicaps at all: tournament golf is played off scratch, with everyone measured against par.