In some levels there are little tank-like units that remain stationary. These are the units that shoot out the lightning and repair structures. They cannot move on their own, but you can pick them up to place them anywhere and they will slowly repair any nearby units or structures. If you pick one up and fly around with it, whenever you take on damage it will fix your ship. Be careful though, because any enemies that may shoot at you from below could hit it, and they cannot repair themselves. However, sometimes it better to leave them near your base so they will repair things as needed.