Anita W.’s question answered:
That is an outflow valve, found on all cabin-pressurized airplanes.
In the air: To keep a cabin pressurized, air is constantly coming into the cabin via engine air. Before the air is delivered to the cabin it’s processed through a series of stages, until perfect for human breathing and temperature comfort distribution.
After cycling and recycling through the cabin for a few minutes that air goes overboard, being replaced with new fresh air. This is a constant cycle.
The outflow valves (typically one forward and one aft on the airplanebody), regulate how much air is allowed to leave the airplane, to maintain correct pressurization so that we can breathe normally and to stay in comfortable temperature surroundings while flying.
On the ground (as in the pictures) the outflow valve is fully open, since the plane is not pressurized. The higher we climb the more the outflow valve will close to maintain correct pressurization. On landing the outflow valve will go fully open again. This ensures the airplane is not pressurized on the ground.
A ground-pressurized airplane would not allow the cabin doors or overwing exits to open to exit the airplane.
You can read more on that HERE, if desired.
