A Privateer was a private warship authorized by a country's government by letters of marque to attack foreign shipping. Strictly, a privateer was only entitled by its state to attack and rob enemy vessels during wartime.
Privateers were part of naval warfare of some nations from the 16th to the 19th century. The costs of commissioning privateers was borne by investors hoping to gain a significant return from prize money earned from enemy merchants.
In Evony, your army will plunder a city's resources, if your troops are victorious in the battle. Without a warehouse, all resources are available for plunder; up to the load limit of the attacking army. A warehouse will protect whatever goods are stored within it.
Privateering gives your attacking army the chance to plunder those annoying Warehouses. Without Privateering research, none of the goods in a warehouse are available to you. A higher research level allows you to steal more from a Warehouse. Each research level increases your plunder from a warehouse by 3%.
Note that your Acadamy must be upgraded to Level 10 before you can start researching Privateering.