What it measures
Spin rate is the total speed of the ball's rotation, in rpm, measured just after it leaves the face. For normal shots almost all of it is backspin, with the top of the ball rotating back toward the golfer. Any tilt of the spin axis, which creates curve, is shown separately as spin axis.
It is a total, so a high number does not say which way the ball is curving. The meaning is the same for right and left handed golfers.
Typical numbers
PGA Tour players average about 2,545 rpm with the driver and about 6,200 rpm with a 6 iron. A tour 7 iron spins roughly 7,000 rpm, and a full pitching wedge around 9,000 to 9,500 rpm. LPGA averages are a little lower, about 2,500 rpm with the driver and 5,900 with a 6 iron.
Everyday golfers usually spin the driver more. Male amateurs average from about 2,900 rpm for scratch players to about 3,300 rpm for the average 14 handicap. Iron spin for everyday golfers varies widely with strike quality, from well under tour numbers on thin shots to very high on steep, glancing blows.
What changes it
Spin loft is the main factor: the larger the gap between the face's delivered loft and the club's direction of travel, the more spin. More loft or a steeper downward blow raises spin; less loft or hitting up lowers it. Club speed multiplies the effect, so faster swings make more spin at the same spin loft.
Strike location matters a great deal. On a driver, a low strike adds spin and a high strike reduces it through vertical gear effect. Grooves, a clean face and ball, the ball model and anything trapped between face and ball (grass, water, or a fat strike off the mat) change friction and therefore spin.
What it changes
Backspin creates lift. With too little, the ball falls out of the air early and runs; with too much, it climbs steeply, loses energy and lands short, especially into wind. For the driver, lower spin with higher launch usually gives the longest carry and roll at a given speed. For irons and wedges, spin helps the ball land steeply and stop on the green.
Spin also affects how much the ball curves for a given spin axis tilt. A slice or hook with high spin bends more than the same tilt with lower spin, so extra spin magnifies directional misses.
How to use it
Know your target window for each club rather than chasing a single number. For many everyday driver speeds, roughly 2,200 to 3,000 rpm is efficient; slower swings tend to need the upper end. Iron spin near the club number times 1,000 (about 7,000 rpm for a 7 iron) is a common rule of thumb at tour speeds, and less is normal at slower speeds.
Bay drill: in the shot lab, hold launch and speed steady and slide spin up and down to see how carry and peak height respond. Then hit five drivers, noting where on the face you struck each one with face tape or spray, and compare spin to strike location. Moving strikes slightly higher on the face is often the fastest way to lower driver spin.