Prism is a framework for building maintainable and testable XAML applications in WPF and Xamarin Forms. It provides implementations of design patterns like MVVM, dependency injection, and commands. Prism's core functionality is shared across platforms through a .NET Standard and .NET library. The document discusses downloading and setting up Prism, including installing system requirements, extracting the source code, and using the Prism Template Pack in Visual Studio. It also mentions including productivity tools and a demo.