Mono это виртуальная машина для выполнения C# программ на платформах (Android, iOS)

• компилятор языка C# (mcs)
• среду выполнения промежуточного кода (CIL кода) с поддержкой JIT (Just in Time) и mint (без поддержки JIT)
• отладчик

Runtime:
The Mono for Android runtime is an application that runs on the Linux kernel in the Android stack.
It interprets the Mono byte code and handles communication with the Dalvik runtime for calls to native Android APIs.

iOS – C# is ahead-of-time (AOT) compiled to ARM assembly language. The .NET framework is included, with unused classes being stripped out during linking to reduce the application size. Apple does not allow runtime code generation on iOS, so some language features are not available (see Xamarin.iOS Limitations ).

Android – C# is compiled to IL and packaged with MonoVM + JIT’ing. Unused classes in the framework are stripped out during linking. The application runs side-by-side with Java/ART (Android runtime) and interacts with the native types via JNI (see Xamarin.Android Limitations ).

Windows – C# is compiled to IL and executed by the built-in runtime, and does not require Xamarin tools. Designing Windows applications following Xamarin’s guidance makes it simpler to re-use the code on iOS and Android. Note that the Universal Windows Platform also has a .NET Native option which behaves similarly to Xamarin.iOS' AOT compilation.

Что такое Mono? (в интернете)
1) Основные принципы Mono...

2) Part 1 – Understanding the Xamarin Mobile Platform...

3) Официальный сайт http://www.mono-project.com/docs/about-mono/

4) Книга "Professional Android Programming with Mono for Android and .NET/C#"

5) https://www.ibm.com/developerworks/ru/library/os_mono1/

6) https://nklug.org.ua/lg/lg84/ortiz.html