检验验证码的处理

下面的语句是啥意思？新手望详解
#region 检查验证码
        if ((Session["CheckCode"] != null) && (Session["CheckCode"].ToString() != ""))
        {
            if (Session["CheckCode"].ToString().ToLower() != this.authcode.Value.ToLower())
            {
                this.authcode_error.InnerText = "验证码错误";
                Session["CheckCode"] = null;
                return;
            }
            else
            {
                Session["CheckCode"] = null;
            }
        }
        else
        {
            Response.Redirect("Login.aspx");
        }