lock语句问题 请高手帮忙

Question

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
namespace ConsoleApplication3
{
    class Program
    {        static void Main(string[] args)
        {
            Class1 cs = new Class1();
            Thread th = new Thread(new ThreadStart(cs.test1));
            Thread th2 = new Thread(new ThreadStart(cs.test2));
            th.Start();
            th2.Start();
            Console.ReadKey();
        }    }
    public class Class1
    {
        public void test1()
        {
            lock (this)
            {
                Console.WriteLine("test1");
                Thread.Sleep(2000);
            }
        }
        public void test2()
        {
            lock (this)
            {
                Console.WriteLine("test2");
                Thread.Sleep(2000);
            }
        }
    }
}
这个程序中：
Thread th = new Thread(new ThreadStart(cs.test1));
            Thread th2 = new Thread(new ThreadStart(cs.test2));
这两句是什么意思 看不懂 请指教 请尽量详细 十分感谢