The document discusses developing mobile apps using C# for both iOS and Android platforms. It introduces Xamarin, which allows writing apps in C# that are then compiled to fully native iOS and Android apps. Xamarin allows for code sharing between platforms while still using each platform's native UI and APIs, avoiding the downsides of writing multiple versions or using a lowest common denominator approach. Developers can use C# and .NET to write apps that run on over 2.6 billion devices supporting iOS and Android.