From the Help:

An adapter is a component that lets users integrate with an application built upon a specific platform (Windows, web, and so on) by interrogating the application to produce metadata describing the application. Adapters have accessible properties, methods, and events.

So an adapter is what you use to communicate with an application.  You configure it to contain the controls you are interested in working with using a process called interrogation.  It is covered in the Essentials course on Pega Academy.