A Customs House is a building that can be placed on field tiles after the Trade technology has been researched. Once placed, it increases resource production by 2 stars for each adjacent port. It costs 5 stars to build a Customs House. Each city may only contain one Customs House.
Note that it does not provide population like the other tier 3 "counting" buildings. Forges give you population for each Mine, Sawmills give population for each Lumber Hut, Windmills give population for each Farm, but the Customs House produces resources instead.