This has come up and been answered a couple of times. It protects against physical missile attacks.
It was this answer. It does not included magical and not solid (Fire, lightening, gas or acid) ranged attacks. It does include thrown knives, hatchets, boomerangs, goblins. It also includes fired arrows, bolts from a crossbow, spears and squirrels (has come up).
We have ruled that it does not work against cannons and field guns.