Scripting backend is a framework that powers scripting in Unity. As of today, Unity supports three different scripting backends depending on target platform: Mono, .NET and IL2CPP. Windows Store, however, supports only two: .NET and IL2CPP.
You can switch between selected scripting backend in player settings:
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.