Plunder can mean stolen goods or money obtained illegally, or the act of taking those things. A burglar might plunder a jewelry store and then sneak off with her plunder.
Plunder is an old Middle High German word that originally meant "household goods and clothes": in other words, your stuff. During the Thirty Years' War (1618-1648), English speakers acquired this word while fighting in the land that is today Germany, but with the added meaning of taking the plunder as, well, plunder. As both a noun and verb, a synonym for plunder is loot.