Resupply will cause the unit to seek out the closest empty Airpad, Airplane Plant or Reef for health or ammo. Note although that the chosen Airpad is not changed automatically upon the availability of another Airpad, then unit will take other units coming to the landing spot and will avoid Airpads that have been "Booked full".
Resupply can be issued to and only to air units, namely all gunships, all planes, all drones and Athena. It can only be issued to units that are either damaged or need rearming. It does not have a default hotkey but one can be bound in the settings menu. If a unit has been issued the Resupply command but does not need rearming or repair (for instance, issuing the command to a damaged unit, then issuing the Wait Command and repairing the unit back to full health before issuing Wait again) the unit will still seek out an Airpad but will take off the moment it hits the pad. It can also be issued by right clicking on a Airpad or other landing zone and is issued automatically if a bomber has a empty queue and has no ammo.
Normally it is impossible to issue the resupply command to drones due to their uncontrollable nature, though it can be proven that drones can also accept the resupply command through Cheats
Resupply is excellent for replenish the ammo of bombers or the health of planes. it can also act as a manual retreat for fighters and gunships.