'Air gap' here refers to the space at the rim of a small engine's flywheel between the flywheel's embedded permanent magnets, and the poles of the adjacent ignition coil's laminated armature. Air gap is meant to be small -- typically a few thousandths of an inch.
Air gap is a fairly critical adjustment in small engine magneto ignition systems. The smaller the air gap, the better will be the magnetic coupling effect between the flywheel's permanent magnets, and the coil armature's poles. Reliable ignition at cranking speed demands an air gap that's well within specification. Unfortunately, meeting published air gap specifications is fraught with difficulty.
Air gap adjustment is complicated by the magnetic attraction of the flywheel's permanent magnets to the coil armature's poles. Steel feeler gauges are not useable, and non-ferrous metal feeler gauges are awkward because of their stiffness.
The best gauge I know of for the adjustment is a piece of ordinary, 20 lb. office paper. The paper is about 0.004" thick. Here's the method for using it.
1) Remove the spark plug so the engine is easy to turn by hand.
2) With the coil backed off from the flywheel, get the flywheel's magnets lined up with the coil armature's poles.
3) Insert a double thickness of paper evenly into the air gap, like so.
4) Loosen off the coil. The coil will be strongly attracted to the flywheel's magnets through the paper.
5) Tighten the coil's fasteners securely and extract the 'gauge'.
You'll end up with a tight, nominal 0.008" air gap -- about as good as it gets.
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