What is the meaning of Back_channel?

The smaller of two channels in a river that diverge to form an island.

A secondary channel of activity, typically in parallel with primary channels and somewhat independent of them.

An unofficial communications channel used to make informal or subversive negotiations.

A real-time online discussion that occurs simultaneously with a verbal presentation.

A gesture or short prompt by which a listener invites a speaker to continue.

Source: wiktionary.org