Luft

Luft (German for 'air') is a prophylactic pawn push in front of a castled king to give it a flight square off the back rank. When all the squares in front of your king are blocked by your own pawns, a single check on the back rank can be checkmate. Playing h3 (or g3, or the queenside equivalent) costs one tempo and removes the whole back-rank vulnerability.

Source for further study: classic king-safety lore (back-rank prophylaxis).