Microsoft has long been suffering from a deficit of Windows Phone applications. In a major move at Build 2015, Redmond announced two separate projects to target Android and iOS applications and allow them to run on or be ported to Windows.
Targetting iOS applications, Project Islandwood is aimed at easier porting and integration of Objective C. Using this middleware, Project Islandwood is able to provide the APIs that the iOS applications expect. This allows apps to be easily ported with minimal work and recompiling. One of the notable examples is King and their popular game Candy Crush Saga which only had be tweaked a bit to run on Windows. Since the app is now compiled for Windows, it can run on anything from Windows Phone/Mobile to full on Windows 10.
On the Android side of things, there is Project Astoria. Astoria exists as a runtime layer between Windows and the application, allowing any old APK to run without modifications or being recompiled. This runtime layer is limited to Windows Mobile/Phone only however so desktop Windows 10 users are out of luck. Applications relying on Google Mobile services which are closed source APIs will are not work without modification.
Microsoft has been taking their app deficiency seriously as both Windows Phone and Windows RT have suffered from low numbers. Whether or not easy porting and a more unified customer base can attract developers remains to be seen.