It syncs the email on the device and the pc in box. For example, when you open a message on the device and then look in Outlook on your pc, the message is shown as opened. If you delete it on the device, you have an option to delete on the handheld, or both the handheld & mailbox.
Match the emails on your Blackberry with the emails in your account; i.e. what's read/unread, deleted/undeleted, etc, etc...
just for the record, thats what i thought, but when i tried it on my 8330 with VZW to my gmail account, stuff i deleted in gmail wouldnt delete on my berry, even with the sync deleted items box checked on the BIS page. or does it only work from berry to email