There are four (4) sides of a ship. The Stern is the rear of a ship. The fastest way to sink a ship, is to damage the ship through it's stern. This tactic is called: "sterncamping". The stern has about 25% (1/4) integrity of your default broadside armor, so always try to keep your stern protected!
Since the default integrity of the stern is low, it's generally not worth to take Fittings to increase that value.
For more information: Hull.
This information is provided by the update that PotBS Wikia is undergoing, by Captain Vuur.