There is a LOT of information available at Microsoft's support
knowledgebase site (http://www.microsoft.com/support). Select "Outlook
2003" as the product and search on "cached exchange" (or search on the
KB article #870926).
Cached Exchange mode is an improved method of connecting to the Exchange
server with OL2003 that stores a full copy of your mailbox locally and uses
that before it goes to the server for information. If you disconnect from
the network, you don't need to restart Outlook to keep working.