解释一下PostBack是什么？

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace Web1.WebFromTest123
{
    public partial class PostBackTest : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            //1、首先判断是否是提交提交会来的页面，如果是，就不进行初始化操作
            if (!IsPostBack)
            {
                ListItem Beijing = new ListItem("北京", "bj");//显示的值，，编程中用到的value
                ddlProvince.Items.Add(Beijing);
                ddlProvince.Items.Add(new ListItem("河北", "hb"));
                ddlProvince.Items.Add(new ListItem("河南", "hn"));
                ddlProvince.Items.Add(new ListItem("湖南", "hunan"));
            }
        }
        //2、如果省的下拉列表中的值发生了变化，就会触发postback这个js方法
        protected void ddlProvince_SelectedIndexChanged(object sender, EventArgs e)
        {
            //3、在加载城市之前，把第一次加载的项清除、
            ddlCity.Items.Clear();
            //4、获取当前选择的那个省的value
            string prove = ddlProvince.SelectedValue;
            if (prove == "bj")
            {
                ddlCity.Items.Add(new ListItem("朝阳", "hn"));
                ddlCity.Items.Add(new ListItem("保定", "hn"));
                ddlCity.Items.Add(new ListItem("丰台", "hunan"));
            }
            else if (prove == "hb")
            {
                ddlCity.Items.Add(new ListItem("北京", "hb"));
                ddlCity.Items.Add(new ListItem("石家庄", "hn"));
                ddlCity.Items.Add(new ListItem("唐山", "hunan"));
 
            }
            else if (prove == "hn")
            {
                ddlCity.Items.Add(new ListItem("开封", "hb"));
                ddlCity.Items.Add(new ListItem("郑州", "hn"));
                ddlCity.Items.Add(new ListItem("新乡", "hunan"));
            }
        }
    }
}