What it measures
Imagine the clubhead's arc near the ball as a tilted hoop. Where that hoop meets the ground forms a line, and swing direction is the angle between that line and your target line, in degrees. It describes the whole arc from about knee height in the downswing to about knee height in the follow-through, while club path is the club's direction at the single instant of impact. Like every directional number, it is measured against the target line, so if you aim somewhere else in the bay, it shifts with your aim.
For a right-handed golfer, a positive number means the arc points right of the target, which coaches often call swinging from underneath or in to out. A negative number means it points left, often called over the top or out to in. Zero means the arc is aimed straight down the target line. For left-handed golfers the picture is mirrored.
Typical numbers
There is no standard tour average for swing direction, because it varies with each player's attack angle and plane. With irons, good ball strikers often show a slightly negative number because hitting down pushes the path to the right, and a little left in the arc brings the path back toward neutral. About 2.5 degrees left is a common target for straight iron shots, and a useful guide for a 6 iron is a negative swing direction at least half the size of the attack angle.
With a driver hit on the way up, the opposite applies: a neutral path usually requires an arc pointed a few degrees right. Everyday golfers who slice commonly read several degrees left with most clubs, but the spread is wide.
What changes it
Alignment is the biggest lever. The arc tends to follow where the shoulders point, so shoulders open to the target pull the number left and closed shoulders push it right. A ball position far forward often opens the shoulders at address. In the downswing, unwinding the shoulders and arms first throws the club outward and left, while letting the arms drop and the body lead swings the arc to the right.
What it changes
Swing direction sets up club path together with attack angle and swing plane. For a right-handed golfer, the path roughly equals the swing direction, shifted right when you hit down and left when you hit up. The size of that shift is about one degree per degree of attack angle on a flat driver plane near 45 degrees, and around six tenths of a degree on a steeper iron plane near 60 degrees.
Example: a 7 iron with swing direction 3 degrees left and attack angle 4.5 degrees down on a plane near 58 degrees ends up with a path near zero. A driver with swing direction at zero and attack angle 4 degrees up on a 47 degree plane ends up with a path close to 4 degrees left, which favors a fade or slice. Path, combined with face angle, then sets start direction and curve, and swing direction also shows up in low point side.
How to use it
Always read swing direction alongside attack angle and club path rather than on its own. If your path is left with the driver, the fix is often to hit more up and swing the arc slightly right at the same time, not one or the other.
Bay drill: lay one alignment stick on the mat along the target line and a second one a few degrees right of it. Tee the ball higher and a touch forward, swing along the second stick while keeping the face aimed at the target, and watch swing direction move positive and club path move toward zero. With irons, a club held across your shoulders at address will show whether open shoulders are behind a strongly negative reading.