Why we use static before instantiating a class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Construct
{
    class Program
    {
public static Base b = new Base();
        static void Main(string[] args)
        {
            b.add();
        }
    }

    public class Base
    {
        public void add()
        {
            Console.WriteLine("I Invoked");
        }
    }
}

I am a beginer in programming.....

In this case the bolded font....can any one explain the use of "static" keyword  while instantiating a class (keep reference variable as  stataic....what is the use and which scenario we use this) or its working details....plzzzz