ifconfig is an extremely useful command for networking. It is also quite dangerous, as the wrong configuration can bring a network interface down. It is, however, safe to execute the command without any arguments. It lists all the network interfaces that are currently set up, and reports the status of each one.
Most hosts have at least two interfaces. eth0 is a common name for an Ethernet network interface. lo is a “loopback” network interface that permits very high speed and efficient communication for a host to itself.
It may sound strange to have such an interface. However, this interface is very useful for programs that communicate with another program via sockets, even if both programs are active on the same host.