If you would otherwise be hit by a missile, having the Air Shield trait has a chance to protect you from that. The little force field (or air bubble) has a listed point value, which is the percent chance for the missile to be negated.

Unfortunately for the Air Shield, any missile that ignores Shields will bypass it. Most of these are spells that don't even look like missiles, such as the bundle of sparkles from a Mind Burn or the minor lightning strike from Lightning Bolt. Dominions 5 also had all Area-of-Effect projectiles (such as Fireballs) bypass Air Shields.