When you're visiting the master visitor list (Lists > Visitors), when you select a visitor you'll notice a button to Set VIP and Add VIP Notification.
Setting a visitor as a VIP will simply flag them to the receptionist or guard when they check in. This may signal for the receptionist to be especially polite for example.
The Add VIP Notification lets you add users who will be notified (through email) whenever that visitor arrives or books a visit. This can be useful to make sure that there is someone to meet them in the lobby, or to arrange lunch for example.
Next Topic: Visitor Management