A default route, also known as the gateway of last resort,
is the network route used by a router when no other known
route exists for a given IP packet's destination address.
All the packets for destinations not known by the router's
routing table are sent to the default route. This route
generally leads to another router, which treats the packet
the same way: If the route is known, the packet will get
forwarded to the known route. If not, the packet is
forwarded to the default-route of that router which
generally leads to another router. And so on. Each router
traversal adds a one-hop distance to the route.

Once the router with a known route to an host destination is
reached, the router determines which route is valid by
finding the "most specific match". The network with the
longest subnet mask that matches the destination IP address
wins.

The default route in IPv4 (in CIDR notation) is 0.0.0.0/0,
often called the quad-zero route. Since the subnet mask
given is /0, it effectively specifies no network, and is the
"shortest" match possible. A route lookup that doesn't match
anything will naturally fall back onto this route.
Similarly, in IPv6 the default address is given by ::/0.

Routers in an organization generally point the default route
towards the router that has a connection to a network
service provider. This way, packets with destinations
outside of the organization's local area network
(LAN)?typically to the Internet, WAN, or VPN?will be
forwarded by the router with the connection to that provider.

Host devices in an organization generally refer to the
default route as a default gateway which can be, and usually
is, a filtration device such as a firewall or Proxy server.