专业人士，帮我解释解释，谢谢了，急用！

public partial class lyb : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (Session["admin"] != null)
        {
            admin_go.Text = "退出管理";
            admin_go.NavigateUrl = "exit.aspx";
        }
        if (!this.IsPostBack)
        {
            string ToPage = Request.QueryString["ToPage"];
            if (ToPage == null)
            {
                ToPage = "1";
            }
            if (!StrRegExp.IsID(ToPage))
            {
                ToPage = "1";
            }
            this.Bind_rptList(Convert.ToInt32(ToPage));
        }
    }