This is a passive skill and is gained automatically by the Pirate Privateer class via the Uncharted Waters expansion pack.
When you disable a vessel in combat you will notice there is a button that allows you to Plunder the vessel, doing so will give you an extra loot item depending on the type of vessel and it will then cause the vessel to disappear so you can fight another one.
The Plundering skill does not go up, and will remain at the value it was when you started.
Plundered loot can be sold at a port as if it was ordinary loot you gained from disabling the vessel in the first place.