A "thank you page" is a small courtesy you extend to your viewers.  You can thank them for visiting, for taking the time to fill out a form, etc.  (You can set this letter up with field merge files that personalize the letter for your customer - see the example on this page)

One important thing you will want to do is to remove the thankyou.htm page from any navigation system or site map.  This page should only be seen by certain viewers, i.e. those who submit one of your forms so it is not necessary to have it linked to the rest of your pages.