An air support unit is a unit that supports air units by refueling and repairing them.
Air staging facilities (UEF version, Cybran version, Aeon version) and the Cybran cruiser provide a platform where aircraft can land. Air units that patrol close enough to them will automatically land to refuel or repair battle damage. They can also be explicitly ordered to land on these units.
A carrier is also an air support unit. When aircraft are loaded into carriers, the carrier repairs and refuels them. The carrier protects the aircraft from attack, though if the carrier is destroyed, all units loaded inside it are also destroyed.