What is a Default Route
A default route is a router setting. It tells the router where to send data packets if the router is unsure how to get to the destination address.
Routers work by having a table of information which links certain IP address blocks to certain ports on the router. If IP address 10.2.2.2 is linked to port 2 of the router, then all packets with a destination to 10.2.2.2 go out port 2, and port 2 only.
A default route is the last possible place the router checks to see where to send a packet. If the packet's destination address is not in the router's routing table, then the router defaults to its default route.
An example default route would say that the default for all routes goes to port 3. So when the router cannot find any other information about a source IP address, and it has no idea where to send the data packet, it sends it out port 3. Like the name implies, a default route is the final default for sending traffic.
Again, the default route is only used if there are no other routes for a certain destination IP address in the router's routing table.
Why are Default Routes a Good Thing?
Say you run a small office network that has a single connection to the Internet. How do you setup the router? Easy, you set all the local IP blocks on the router. This allows different parts of the office to reach each other. Then you set a default route on the port that connects to your ISP.
This way a router knows where to send all inter-office traffic. Any traffic destined to somewhere other than the inter-office ends up getting sent to the default route, which is the port connecting to the ISP, which is the Internet. This makes sense, as everything other than the office is outside of the office.
If default routes did not exist every router would have to have a full routing table of the whole Internet. This is a waste of memory on the router. Default routes make configurations simple and routers run faster because of it.