It's a complicated mathematical computation that is determined by the best 10 of your last 20 scores.

The handicap is then applied to a course slope/rating, giving you your course handicap. Then, with this number, you figure out how many strokes you take through Equitable Stroke Control (ESC).

This is for the RCGA and USGA. I believe the rest of the world runs off a 'standard scratch score (SSS)' system.