What it measures
Impact height is the up and down distance from the center of the clubface to where the ball made contact, measured in millimeters or inches. Strikes above center read as positive and strikes below center read as negative. It reads the same for right and left handed golfers.
Typical numbers
There is no tour average that matters for everyone. With driver, many good players strike slightly above center, because a little high on the face tends to give higher launch and lower spin. With irons the best contact is usually near center or just below, since the ball sits on the ground and the sweet spot is lower on the face.
Everyday golfers often show a wide vertical spread. Driver strikes low on the face and thin iron strikes near the bottom grooves are the most common misses.
What changes it
Tee height is the biggest factor with driver: a higher tee usually moves contact up the face, a lower tee moves it down. Attack angle matters too. Hitting up moves contact higher, and hitting down with a driver tends to move it lower. With irons, low point position and posture changes during the swing control it: a low point behind the ball or standing up through impact produces thin, low strikes.
What it changes
Away from center, ball speed and smash factor drop. Driver heads also have vertical gear effect. A strike above center tends to launch higher with less spin, and a strike below center tends to launch lower with more spin. The difference between a high and low driver strike can be several hundred to over a thousand rpm, and it varies with head design. Very high strikes can still lose a lot of speed, so the goal is slightly above center, not the top edge.
With irons, low strikes come out low and short with less speed, and high strikes reduce speed and often add some launch.
How to use it
If your driver spins too much and launches low, check impact height before changing your swing. Moving contact up a little can lower spin and raise launch at the same time.
In the bay, run a tee height test: hit five drives each at a low, medium and high tee, and compare impact height, launch angle, spin rate and carry. Pick the height that gives the best combination. For irons, try placing a thin towel a few inches behind the ball and making contact without touching it, which encourages a forward low point and more centered strikes.