What is the loopback interface lo0 on Macs?
Loopback is a virtual interface that is an integral part of all TCP/IP network interfaces on all generally available operating systems. From Wikipedia:


  Any traffic that a computer program sends to a loopback IP address is
  simply and immediately passed back up the network software stack as if
  it had been received from another device.


Since loopback interfaces don't send data anyplace but back from where they came you will not generally find ways to hack a system using loopback. That does not mean it can't or has not been done but it is not something to generally worry about.