Wall Heater Exhaust Vent
There's a good chance this vent is for a wall
heater and it is on the other side of the wall.
You will not know for sure until you go inside the house and finish your home inspection.
These types of wall heater vents will need to be at least 10 feet away from a window that opens.
Check your local building codes for the minimum distance they require from a operable window.
The reason for this is to prevent exhaust fumes from entering the living area.