浙西大环线:找高手告诉我这段代码每一行的意思

<%
// *** Validate request to log in to this site.
var MM_LoginAction = Request.ServerVariables("URL");
if (Request.QueryString!="") MM_LoginAction += "?" + Server.HTMLEncode(Request.QueryString);
var MM_valUsername=String(Request.Form("admin"));
if (MM_valUsername != "undefined") {
var MM_fldUserAuthorization="";
var MM_redirectLoginSuccess="longin_1.htm";
var MM_redirectLoginFailed="longin_2.htm";
var MM_flag="ADODB.Recordset";
var MM_rsUser = Server.CreateObject(MM_flag);
MM_rsUser.ActiveConnection = MM_books_STRING;
MM_rsUser.Source = "SELECT userid, userpasswd";
if (MM_fldUserAuthorization != "") MM_rsUser.Source += "," + MM_fldUserAuthorization;
MM_rsUser.Source += " FROM userinfo WHERE userid='" + MM_valUsername.replace(/'/g, "''") + "' AND userpasswd='" + String(Request.Form("password")).replace(/'/g, "''") + "'";
MM_rsUser.CursorType = 0;
MM_rsUser.CursorLocation = 2;
MM_rsUser.LockType = 3;
MM_rsUser.Open();
if (!MM_rsUser.EOF || !MM_rsUser.BOF) {
// username and password match - this is a valid user
Session("MM_Username") = MM_valUsername;
if (MM_fldUserAuthorization != "") {
Session("MM_UserAuthorization") = String(MM_rsUser.Fields.Item(MM_fldUserAuthorization).Value);
} else {
Session("MM_UserAuthorization") = "";
}
if (String(Request.QueryString("accessdenied")) != "undefined" && true) {
MM_redirectLoginSuccess = Request.QueryString("accessdenied");
}
MM_rsUser.Close();
Response.Redirect(MM_redirectLoginSuccess);
}
MM_rsUser.Close();
Response.Redirect(MM_redirectLoginFailed);
}
%>