What it measures
Attack angle describes the up or down direction of the clubhead's travel at impact, in degrees. It is taken at maximum compression, the instant the ball is most squashed against the face, and measures it against level ground rather than against the slope of the hitting surface.
A negative number means the club is still descending when it meets the ball, which is normal for irons and wedges played off the turf or a mat. A positive number means the club has passed the bottom of its arc and is rising, which is common with a driver on a tall tee. Zero means the club is moving level. The sign convention is the same for right and left handed golfers.
Typical numbers
PGA Tour players average about -0.9 degrees with the driver and about -3.7 degrees with a 6 iron. LPGA Tour players average about +2.8 degrees with the driver, hitting up to make up for lower club speed, and about -2.3 degrees with a 6 iron. Tour players typically hit down a little more as the clubs get shorter, so a 7 iron sits around -4 degrees and full wedges around -4 to -5 degrees, though these vary by player.
Among everyday male golfers, driver attack angles average from about -0.9 degrees for scratch players to about -2 degrees for bogey golfers, so most amateurs still hit slightly down on the driver. With irons, everyday golfers range widely, from shallow or even slightly upward strikes (often a sign of a thin or fat contact pattern) to very steep blows of -7 degrees or more.
What changes it
Ball position is the biggest lever. Moving the ball forward in your stance lets the club reach the ball later in the arc, after it has bottomed out, so the angle rises. Moving it back makes the angle more negative. Tee height works with ball position on driver shots, since a higher tee makes an upward strike possible without catching the ground.
Body motion matters too. Keeping weight and pressure moving toward the lead side and the chest over the ball tends to steepen the angle with irons. Tilting the spine away from the target at address and through impact makes it easier to hit up on a driver. Swing direction also has a small effect: an out to in swing tends to be steeper, an in to out swing tends to be shallower.
What it changes
Attack angle mainly affects spin loft, the difference between where the face points (dynamic loft) and where the club is moving. For a given dynamic loft, hitting up shrinks spin loft, which lowers spin and usually improves smash factor. That is why tour long hitters and fitters chase a positive attack angle with the driver: the same club speed produces a higher launch with less spin and more carry.
With irons, hitting down moves the bottom of the swing in front of the ball, which is what produces ball first contact. Too steep, though, and spin climbs, launch drops, and divots get deep. Attack angle also tilts the D plane, so a downward blow with an in to out path, or an upward blow with an out to in path, slightly changes the effective path and curve of the shot.
How to use it
Look at averages over five to ten swings, not single shots. With the driver, try teeing the ball higher and moving it toward your lead heel, then watch whether the number climbs toward zero or positive while launch rises and spin falls. With irons, a steady number between about -2 and -5 degrees along with solid contact is a good target for most golfers.
Bay drill: hit three drivers at your normal setup, then three with the ball one ball width further forward and teed a little higher, then three back at normal. Compare attack angle, spin rate and carry for each group in the shot lab. If spin drops and carry rises with the forward position, you have found free distance without swinging harder.