What is the Double.NaN field in C#?

// use System
using System;
// create class
class DoubleNaN
{
    // main method
    static void Main()
    {
      // print field value
      Console.WriteLine(Double.NaN);
      // perform arithmetic operations
      double d1 = Double.NaN + 2;
      double d2 = Double.NaN / Double.NaN;
      double d3 = 23.34 * Double.NaN;
      // print values
      Console.WriteLine(d1);
      Console.WriteLine(d2);
      Console.WriteLine(d3);
    }
}