Xamarin.Forms is a cross-platform UI toolkit with which developers can easily create native user interface lay-out that can be shared among Android, iOS and Windows Phone. Its advantage is that it brings together the user interfaces of each platform - the individual control systems and navigation metaphors - into one common layer, which can be used to build applications for iOS, Android and UWP with both a shared application layer as well as a user interface layer.
What does a Xamarin developer do?
Xamarin developers are very familiar with the Xamarin platform. Xamarin is part of the .NET ecosystem, so a Xamarin programmer knows their way around here as well. They have knowledge of making custom components that use native code. They also know a lot about Patterns and best practises for Cross Platform Mobile Development. Furthermore, thay understand how Data Bindings and Effects can be used in Xamarin.Forms to offer a beautiful, consistent app for multiple platforms.