This question has a better answer here at webmasters stack-exchange site.
More information, which is also cited in the above link, is on httpwatch
According to the article:
There are a number of situations in which Internet Explorer needs to check whether a cached entry is valid:
- The cached entry has no expiration date and the content is being accessed for the first time in a browser session
- The cached entry has an expiration date but it has expired
- The user has requested a page update by clicking the Refresh button or pressing F5