Most PGA Tour events use a scoring system based on stroke play. Every shot counts as one stroke. Then, a player's total strokes are compared to par for the course up to the hole they are on and expressed in terms of the number of strokes under, even with, or over par.
Let's say a golfer who shoots a 65 on a par-72 course in the first round. He is then 7-under par, or -7,  heading into Round 2.
The lowest cumulative score through four rounds wins most PGA Tour events.