This is an exterior termination exhaust duct for a gas wall heater or decorative gas log fireplace.
This is called direct venting.
Direct venting uses a 2-layer pipe running through a hole in the wall behind the interior heating unit.
The outer pipe draws air in from the outside to create combustion for burning the gas in the heating unit... And the inner pipe takes the exhaust gases to the outside.
This design reduces the risks of carbon monoxide buildup within the house.
Note the soot above the termination on the the stucco siding... Not good.
If there is soot or a ‘scorched’ look on the home owner’s brick or siding, then there's a problem... mainly, incomplete combustion taking place in a wall heater or mis-positioned ceramic logs in a direct gas log fireplace.
My recommendation is to have a qualified heating specialist take a look before using the offending gas appliance again.