If you brag that "the world's your oyster," you're using a metaphor from Shakespeare, who knew a thing or two about figures of speech.
Good writers know their way around a metaphor, where you make an analogy between two things to show how one resembles the other in some way. When a character from Shakespeare calls the world his oyster, that's his boastful way of saying that all the riches of the world are his for the taking, like plucking a pearl from an oyster shell. Shakespeare also wrote, "All the world's a stage." Oyster? Stage? Come on, Will, get your metaphors straight!