I was wondering how do you get to Port Plunder and what is it?

You need all of the Uncharted Lands map pieces completed to get to the Omen Islands which is where Port Plunder is. It's the pirate location where the Freyalise pirate ship is docked (you can get an item of free food there every few hours or so, and play a drinking game) and also where the quest Drunk Carl and the Rathskeller Inn shop can be found.