What it measures
Carry is the straight-line distance from the launch point to the spot where the ball's flight crosses back through the same elevation it started from. It is measured in yards or meters.
It counts only time in the air. Bounce and roll are not included, so carry stays the same whether the fairway is soft or firm. Because it is measured at launch level, it also ignores whether the real landing area is uphill or downhill.
Typical numbers
Long-standing PGA Tour averages are about 275 yards for driver, 172 yards for a 7 iron and 136 yards for a pitching wedge. More recent Tour averages are a little longer, around 282, 176 and 142 yards. LPGA Tour players average roughly 218 to 223 yards with driver and around 140 yards with a 7 iron.
Everyday golfers vary widely with swing speed. Many male players carry driver somewhere around 180 to 230 yards and a 7 iron around 120 to 155 yards. Your own averages matter far more than any benchmark.
What changes it
Ball speed is the biggest factor. With driver, each extra mile per hour of ball speed is worth very roughly one and a half to two yards of carry when launch and spin are reasonable. Ball speed comes from club speed and how centered the strike is.
Launch angle and spin decide how well that speed is used. Too low a launch or too little spin and the ball drops early. Too much spin and it balloons and stalls. Thin air at altitude, warm temperatures and a tailwind add carry, while cold air and a headwind take it away. Heavy curve also costs some carry.
What it changes
Carry is the base of Total distance, and the starting point for Side, since the ball's sideways position is read where it lands. It sets which hazards you can fly and how you gap your clubs.
How to use it
Build your yardages from average carry, not your best shot. Hit ten balls with each club, drop obvious mishits, and record the average and the spread. Choose clubs on the course by carry, then think about roll separately.
Bay drill: take three clubs that sit next to each other in the bag and hit five balls with each. Check that their average carries are separated by a fairly even gap, often 10 to 15 yards. Two clubs carrying almost the same distance, or a big hole between clubs, is worth fixing.