The default gateway is the path to other networks. Unless a specific route is defined, all traffic NOT destined for your local subnet goes through the default gateway.
If you have a router, it's address is the default gateway, some (DSL) modems serve as the default gateway.
1) any address is your local subnet does not go through any gateway
2) any address which has a route specified, goes through that gateway
3) any address not otherwise specified goes through the default gateway.
I tried to remain child-like, all I acheived was childish.