收到一封关于退税的信，请大家帮忙看下是什么回事

Base year
                                                              2002
                                                              Payment Period
                                                        July 2003 to June 2004

You applied for the GST/HST credit on your 2002 tax return.
We are unable to determine if you are entitled to any credits because we do not have all of the necessary information.
If you supply us with the following information,we will review it and inform you of the result:
     a Statement of Income,with the amount reported in Canadian dollars,for the period in 2002 when you were not a resident of Canada
Please send this information in writing to the tax centre indicated at the top of this notice.

我是去年11月底登陆的。前两个月已经收到100CAD的退税，现在又收到一封这样的信。大家知道是什么回事吗.GST/HST是什么？
谢谢！