An ASP.NET Web API is an application programming interface that exposes or gives data to other resources without that resource talking to the database directly. Simply put, a Web API's purpose is to speak to (interface with) a different application. The Web API can interact with a person/user or another application on similar or different platforms like the web, desktop applications, mobile devices, etc. It facilitates the use of specific features or data from an application without using a GUI or graphical user-interface.
The API uses database credentials to log into the database. The user, whether a person or a device, can log into the API using other credentials. The user is never exposed to the database credentials creating a layer of security when external resources use the data or features.