Thanks for the replies. It's not combustion air. I have a newer furnace with separate PVC piping for that.
This is an older home (1940) that had duct work added at some point. So some of the ducts are cut into the walls. Basically it's just the space between the wall studs.
I don't know exactly what's what here, but the gap I want to fill is about a one inch gap at the back of the vent. So the vent would only pull air from inside the room, not from outside. I believe the gap opens to the space behind the outside brick. The lower piece is solid wood with an angular cut, which is right above the basement, then the small gap, then some softer particle board or something. I believe I can also feel tar paper thru the gap.
I hope that helps. It's difficult to explain.