What it measures
Spin loft is the angle between two directions at impact: where the clubhead is moving (attack angle and club path) and where the face is pointing (dynamic loft and face angle). It is measured in degrees and is always positive for a normal shot. Because it is an angle between two directions, it reads the same for right and left handed golfers.
A quick estimate is dynamic loft minus attack angle. With 21 degrees of dynamic loft and a 5 degree downward attack angle, spin loft is about 26 degrees. The estimate gets less accurate as face to path grows.
Typical numbers
PGA Tour players average about 14.7 degrees with driver and 24.3 degrees with a 6 iron, and LPGA Tour players about 15.0 degrees with driver and 25.9 degrees with a 6 iron. A 7 iron runs a degree or two higher, and wedges climb into the 30s and 40s and beyond depending on loft and the shot.
Many everyday golfers carry noticeably more spin loft with driver, often high teens or more, from hitting down on the ball or adding loft at impact. Iron numbers vary a lot from swing to swing.
What changes it
Anything that changes dynamic loft or attack angle changes spin loft. Adding loft at impact (flipping the hands, a backward leaning shaft) raises it. Shaft lean toward the target lowers it. With driver, hitting up lowers spin loft and hitting down raises it. With irons, a steeper downward strike without delivering less loft raises it. A large face to path difference also adds a little.
What it changes
Spin loft is the main lever on spin rate. With everything else equal, more spin loft means more backspin and less spin loft means less. It also affects smash factor: lower spin loft produces a more direct, compressed strike and more ball speed, while higher spin loft produces a more glancing blow and less ball speed. Launch angle tends to follow dynamic loft more than spin loft, so two swings can launch similarly but spin very differently.
How to use it
If spin is too high or too low for the club, look at spin loft first, then at which part, dynamic loft or attack angle, is responsible. A golfer who hits up slightly on the driver while keeping loft similar can lower spin loft, cut spin and gain ball speed.
In the bay, try a spin loft ladder with a mid iron: hit three normal shots, three with the ball slightly back and hands ahead, and three with the ball slightly forward. Watch how spin loft, spin rate and smash factor move together. Then aim to repeat the same spin loft on five shots in a row, since consistent spin loft means consistent distance.