What is "out-of-band"

a. 1 (context telecommunications English) relating to activity outside of a defined telecommunications frequency band 2 (context telecommunications by extension English) relating to communication on a different channel, or by a different method, from that of the primary communication channel 3 (context computer security English) relating to communication, such as identity verification, via a method other than the primary means of accessing the software

Out-of-band is activity outside a defined telecommunications frequency band, or, metaphorically, outside some other kind of activity.