Mobile development has always been a big challenge for many developers due to their limited options of platforms to work on, be it Android or iOS. You should study different production environments, Eclipse, objective C, java, etc.
Xamarin is a great and robust paid tool (and very expensive by the way, with the cheapest usage license of $299.90 per year) for developers familiar with the .NET Framework, being able to use the huge repertoire of libraries that this amazing Microsoft framework available with C# to develop mobile applications. Xamarin has a huge documentation to study, with examples of several resources, projects ready for download, community for debate with other developers specialized in the subject.
If you are a .NET web developer and want to venture into a new development scenario, in this case Mobile Development, one of the most sought after areas for projects that is growing every year due to the mobile technological advancement, this is a very good professional tool to study.
More information on the official Xamarin website