Overview
A default gateway is the device that a computer uses to send information to a destination outside its own local network. If a computer doesn't know where to send a packet, it sends it to the default gateway.
Role in Routing
When a device wants to communicate with an IP address that is not on its local subnet, it forwards the traffic to the MAC address of the default gateway. The gateway then uses its routing table to forward the packet further.
Configuration
In most home networks, the default gateway is the IP address of the router (e.g., 192.168.1.1).