Why - A User Story is a one liner, and testing for this should be covered by number of Scenarios , combinations and examples like in BDD. So a User story can be confirmed as passed only if all the scenarios under it are passed.
What - For example a User Story - "As a site visitor, I can filter product listing by price range."
To ensure complete coverage and pass/ confirm this User story for a online store we need to execute multiple scenarios for multiple menu options like:
  1. User should be able to filter product listing by price for "Apparels"
  2. User should be able to filter product listing by price for "Footwear"
  3. User should be able to filter product listing by price for "Accessories"
  4. User should be able to filter product listing by price for "Men's Wear"
  5. User should be able to filter product listing by price for "Women's Wear"
  6. User should be able to filter product listing by price for "Kids Wear"
  7. User should be able to filter product listing by price for "Jewellery"
  8. User should be able to filter product listing by price for "Brands"
  9. User should be able to filter product listing by price for "Gadgets" and so on ...........
To confirm a User Story as Passed, all the scenarios and options available under the User Story should be executed and passed within a short span of time for execution.