What does "server doesn't allow Head requests" mean?

[Basita]

Please can someone explain why my link checker comes up with lots of 405 'Method Not Allowed' error messages for my Amazon affiliate links? It tells me that my server doesn't allow Head Requests and suggests i ask why. I'm quite stuck on this problem... my affiliate links seem to work, but maybe I'm doing something the search engines don't like? Any help would be much appreciated!
Thanks!

http://www.celticsymbols.net