It is the Ram Air Inlet Duct for the cabin air system. It normally acts as cooling air for the packs but can also be used as an alternate vent source in an emergency.
The cooling efficiency of each air conditioning pack is enhanced by
ram air cooling via a heat exchanger. Ram air enters through an inlet
at the abse of the vertical stabilizer and passes through the pack
heat exchangers and then exits overboard. In case of failure of both
packs, ram air can be used as the sole source of ventilation. This is
accomplished by opening the ram-air valve via the RAM AIR switch. Ram
air then enters the occupied areas through the normal vents.