The basic premise behind stub areas in OSPF is that if routers want to communicate to another area they need to send packets to the Area Border Router (ABR), a router that connects OSPF areas. So, by using a default route that points to an ABRs address the routers inside an area can reach all the destinations outside their area without the need to have all the specific routes in their routing...
CCNA, CCNP – OSPF Not-So-Stubby Areas (NSSA)
C