Hello,
out-of-band access in general refers to allowing access of equipment independent of the network connections. An example of Out-Of-Band access would be a serial port connected to a modem that provides dial up access to the router. You can also use the AUX or CONSOLE ports to achieve this.
This Out-Of-Band access will allow a way to access isolated routers during times when network access is not available, which means, simply speaking: it saves you a trip to a remote location.
Does this make sense ?
Regards,
GP