Put simply, a use case is a pre-creation or a symbolic representation of a business scenario. For example, we can represent our login page use case in a pictorial/symbolic representation. In our example, users are trying to log into a system. If the login succeeds, they can enter the system. If it fails, the system informs the users that the login attempt failed. Refer to the following diagram of a login use case:

In the preceding diagram, users called User1, User2, and User3 are trying to enter the system using the login functionality of the application. If the login attempt is successful, the user can access the system. If not, the application notifies the user that login wasn't successful and the user can't access the system. The preceding diagram is much clearer than our actual verbose description, where we are describing this diagram. The diagram is also self-explanatory.