A sufficient quantity of food. Meals on board ship were served to the crew on a square wooden plate in the harbour or at sea in good weather.
Food in the Royal Navy was invariably better or at least in greater quantity than that available to the average landsman. However, while square wooden plates were indeed used on board ship, there is no established link between them and this particular term. The OED gives the earliest reference from the US in the mid-19th century.