An oyster is an animal that lives in the sea, a rough-shelled mollusk. To find a pearl inside an oyster, you first have to find a "pearl oyster" — a rare cousin of the oysters people commonly eat.
You can usually tell you've found an oyster on the beach because its shell is bumpy and irregular, unlike the smoother clams or mussels you might see. There are few kinds of oysters that are edible (both cooked and raw), and many others that aren't. You may have heard the expression "the word ls your oyster," which is one of Shakespeare's many linguistic inventions; he means the world is yours to seize, open, and use as you wish.