A stopper is plug or a cork — it's designed to block the opening of a container by fitting partially inside it. The rubber plug you use to keep your bath water from going down the drain is a type of stopper.
The cork in the neck of a wine bottle is a stopper . Other kinds of stoppers halt or prevent something. A crime stopper keeps criminals at bay, and a conversation stopper is something you say that is so awkward or profound that no one can think of anything to say in response.