The strip of turf your iron lifts out of the fairway on a strike. Replacing it, or filling the scar with sand, is the core piece of course-care etiquette.
A divot is not a mishit, it is evidence of a good one: on a clean iron shot the club meets the ball first and the ground second, so the turf comes out in front of the ball, towards the target. If it starts behind the ball, you went into the ground too early. The etiquette side is simple: put the turf back and tread it down, or fill the scar with the sand mix most courses provide. What people mix up is the divot and the pitch mark — the crater your ball punches into a soft green on landing. That one is never repaired with your foot but with a pitch-mark repair tool: push the edges gently towards the middle, then tap the surface flat. Both are quickly done, and both are the first thing experienced players read you by — on a golf course etiquette opens more doors than a low handicap.