网页的信息头中Expires设置为1981年是什么作用？

yum2014

# curl -I http://jijianzhuyi.tuchong.com/
HTTP/1.1 200 OK
Server: nginx
Date: Thu, 25 Jul 2013 10:03:55 GMT
Content-Type: text/html; charset=utf-8
Connection: keep-alive
Vary: Accept-Encoding
Set-Cookie: PHPSESSID=m9drl7vgdkcgbgrbh96993frk5; path=/; domain=.tuchong.com
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Set-Cookie: site=tuchong; expires=Sat, 24-Aug-2013 10:03:55 GMT; path=/; domain=.tuchong.com; httponly



求问粗体字部分是什么意思？