We support operating systems that not even Microsoft and Apple themselves support anymore, such as macOS X on PowerPC Macs, and RetroArch being available on Windows OSes as far back as Windows 95.
It allows you to install Android Apps on. RetroArch can run on the usual platforms like Windows, Mac OS X and Linux, but it stands alone in that it can support far more platforms beyond just that. An Android emulator is a software application that allows your mobile to imitate Android OS features into your PC.