The expires meta tag tells search engines when the content on your website will expire. The expires meta tag defines the expiration date and time of the document being indexed. If your website is running a limited time event or there is a preset date when your document will no longer be valid, you should include the expires tag to indicate to search engines when to delete your web-page from their database.
The expires tag is commonly used in conjunction with the Revisit Tag as a means to get search engines to re-visit a website every few days. This is commonly used by websites who update their content frequently and want search engines to have a fresh copy of their content.
If you replace or rename a web-page please use a 301 refer to point the seach engines in the right direction. Search engines will automatically replace the old page with the new page.