From the perspective of virtual event participants, a backchannel is a (most often) public space for communication and annotation of the conversation, resource sharing, and a way of communicating without needing a working microphone or quiet space. Examples include Zoom Chat, Slack, and Twitter (using an event hashtag).
From the perspective of the event organizers, a backchannel is a private communication channel via which they can communicate about technical problems, scheduling changes, and other time-senstive issues.
Citation: Center for Scientific Collaboration and Community Engagement. (2022) CSCCE Glossary: Virtual Events. Pratt, Woodley, Ainsworth, Carter, Crall, Elkins, Gauthier, Ihle, Kornahrens, Martinic, Santistevan, Shaikh, Sidik, and Wyatt doi: 10.5281/zenodo.6608609