When a user story is well defined and understood, it is accompanied by acceptance criteria. Acceptance criteria define the boundaries of a user story and help product owners, customers, or business analysts to answer what they need to provide value with the product.
Acceptance criteria help developers identify when to stop adding more functionality and to derive tests for verification and validation purposes. They can also be developed as a story becomes well understood to enable the development team to verify that the solution will meet the user's needs.