Air is the default "absence of item," and is sometimes mined by accident when mining a block at the same time as another player. It disappears from inventory after logging out. The item exists as part of a strategy to prevent item duplication.
How to Obtain
Air is sometimes obtained when mining a block at the same time as another player. It only appears if you have a slot open in your hotbar.
This item has no use, other than to make you wonder why you have a question mark called "air" in your hotbar.
Air was added to prevent duplication.