public sealed class Singleton
{
    Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            return Nested.instance;
        }
    }

    class Nested
    {
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested()
        {
        }

        internal static readonly Singleton instance = new Singleton();
    }
}

I wish to implement Jon Skeet's Singleton pattern in my current application in C#.

I have two doubts on the code

1) How is it possible to access the outer class inside nested class?

I mean internal static readonly Singleton instance = new Singleton(); Is something called closure?

2) I did not get this comment

// Explicit static constructor to tell C# compiler
            // not to mark type as beforefieldinit

what does this comment suggest us?