Microsoft has purchased Xamarin, who currently maintain the Mono project.

This requires a little background. The .NET Framework was announced in 2000, and it quickly became one of the most popular structures to write native applications, especially simple ones. Apart from ASP.NET, which is designed for servers, support extended back to Windows 98, but it really defined applications throughout the Windows XP era. If you ever downloaded utilities that were mostly checkboxes and text elements, they were probably developed in .NET and programmed in C#.

Today, Qt and Web are very popular choice for new applications, but .NET is keeping up.

The Mono project brought the .NET framework, along with its managed languages such as C#, to Linux, Mac, and also Windows because why not. Android and iOS versions exist from Xamarin, under the name Xamarin.iOS and Xamarin.Android, but those are proprietary. Now that Microsoft has purchased Xamarin, it would seem like they now control the .NET-derived implementations on Android and iOS. The Mono project itself, as it exists for Linux, Mac, and Windows, are under open licenses, so (apart from Microsoft's patents that were around since day one) the framework could always be forked if the community dislikes the way it is developing. To visualize the scenario, think of when LibreOffice split from OpenOffice a little while after Oracle purchased Sun.

If they do split, however, it would likely be without iOS and Android components.