What it measures
A golf ball leaves the club with backspin around an imaginary axle. If that axle is level, the ball flies straight; if it tilts, the ball curves toward the low side. Spin Axis is that tilt, in degrees.
Positive values curve the ball right and negative values curve it left. For a right-handed golfer a small negative number is a draw, a large negative number a hook, a small positive number a fade and a large positive number a slice. Roughly minus 2 to plus 2 counts as a straight shot. The axis is set at impact and stays the same through the flight; wind can push the ball but does not change the axis.
Typical numbers
As a rule of thumb, each degree of axis tilt bends the ball about 0.7 yards per 100 yards of carry. That is about 11 yards of curve at 150 yards and about 15 yards at 200 yards for a 10 degree axis. The exact amount varies with speed, spin and launch.
Skilled players usually keep stock shots within a handful of degrees of zero. Everyday golfers who slice often see positive values in the double digits, and by the rule above a 30 yard curve on a 200 yard shot means an axis of around 20 degrees.
What changes it
Face to Path is the main input: open to the path tilts the axis right, closed tilts it left. Spin loft decides how strongly: with low loft, like a driver, one degree of Face to Path tilts the axis much more than with a wedge. That is why drivers curve easily and wedges barely do.
Strike location is the other big factor, mainly with drivers and woods. Gear effect means a toe strike adds draw tilt and a heel strike adds fade tilt, sometimes enough to override Face to Path. Irons show much less gear effect.
What it changes
Spin Axis controls Curve and, with Launch Direction, where the ball finishes (Side). A strongly tilted axis also tends to lower the peak height and carry a little, since part of the lift now pushes the ball sideways instead of holding it up. A hooking ball usually carries less backspin, so it also tends to run farther after it lands.
How to use it
Use Spin Axis to see your true shot shape. If it disagrees with Face to Path, check where you struck the face; D Plane Tilt helps separate swing from strike.
Bay drill: hit ten drivers with foot spray or impact tape on the face. Sort the shots by toe, center and heel strikes and compare their Spin Axis. Then try to hit three shots with the axis between minus 2 and minus 6 and three between plus 2 and plus 6, so you can produce both a small draw and a small fade on purpose.