Xamarin is a framework made by Microsoft to let developers to develop apps running natively on multiple platforms (Windows, iOS, Android).
The beauty about this framework SDK is enabling us to make apps running natively. Our code is developed in C# (so for Windows we need to have Ms. Visual Stdio). It uses native UI for its apps, so once we develop an app for Windows, for example, it's also compilable on Mac OS without code changes. The same for Android.
The framework can be downloaded freely for Windows and MacOS: