A use case describes how a person or another system uses a product to reach a goal. It is like writing down the steps for checking out at a shop, including what happens if a problem occurs.
An online store use case might begin when someone chooses an item and end when they receive an order confirmation. It can include the product's response to missing payment details.
Use cases help teams think through a complete interaction, but they do not dictate the exact screen design or code. Those choices come later.