Advertising into the overlay means a route that is locally learnt on an edge will be sent to the Velo gateway which then reflects it to all other edges in the enterprise (based on the config)
One example - Say we configure a static default route on a hub pointing to some LAN side interface. Now when this route is advertised to the overlay, all other edges that receive this route will install the route in their FIB and the next hop will point to the hub
