What is is.
The date and time when the log server wrote the entry.
What it says.
It tells you - roughly - the time when the URL has been requested. It actually is not defined what event is time stamped by the server, but it's probably the sending of the response.
The time between two requests from the same host might be interpreted as the view time, but - read on.
What may cause trouble.
The date/time field comes in quite a variety of versions, especially regarding the date portions which variies with the locale of the server (i.e., "[01/Sep/2001:12:20:14 +0200]").
Proxy servers can ruin the determination of viewing time. Imagine a hundred users accessing your pages through a single proxy. All requests seem to be coming from the same address. The resulting viewing times will very short, and the presumed single user seems to be hopping wildly all over the web site.