什么是线程？

using System.Threading;
using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.IO;
using System.Linq;
using System.Text;
using System.Xml.Linq;
namespace VSCode
{
    class Program
    {
        static void Main(string[] args)
        {
            Thread tt=new Thread(WriteY);
            tt.Start();
            for (int i = 0; i < 1000; i++)
            {
                System.Console.Write("X");
            }
        }
        static void WriteY()
        {
            for (int i = 0; i < 1000; i++)
            {
                System.Console.Write("Y");
            }
        }
    }
}

temp变量的范围是0~9