Please Explain below mention Code

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

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
1)Explain bold Line what does its mean.
            MainClass obj = new DrivedClass();
            obj.ID = 222;
            obj.Name = "Changed";
            obj.addNum(4, 3);
            Console.WriteLine(obj.Name);
            MainClass obj1 = new MainClass();
            
            //DrivedClass obj2 = new MainClass();
            
            Console.ReadKey();
        }
    }

    public class MainClass
    {
        public Int32 ID;
        public String Name;
       
        public MainClass()
        {
            //do some things
        }
        public Int32 addNum(Int32 a,Int32 b)
        {
            return 3 * 5;
        }
    }
    public class DrivedClass : MainClass
    {
        public DrivedClass()
        {
            ID = 123;
            Name = "Qamar";
        }

        public void add()
        {

        }
    }
}