On ships, the function of the porthole when open is to allow light and fresh air into the vessel and, when closed, is to provide a strong water-tight barrier. They are old fashioned; modern ships often have larger rectanglular but unopenable windows.
A typical ship's porthole consists of a circular Brass opening set into the hull of the Ship, a Glass and brass hinged flap, a solid brass hinged flap and some bolts with hand-operated nuts to lock the flaps in a closed position. With the two flaps open, both Light and Air enter through the hole. With the glass flap shut, only light enters the ship. With both flaps shut, the hole is protected from bad Weather and water entry even if the glass breaks.
Portholes on aircraft, generally do not open; they only provide light to the interior. Portholes used in fortifications generally open to allow a weapon to be used through them.