Plain English Meaning
One switch that turns all the electronic flight equipment on or off together, separately from the rest of the airplane's electrical system.
Full Definition
A single switch in the cockpit that controls electrical power to the aircraft's avionics equipment — radios, navigation receivers, transponder, GPS, and similar electronic instruments — as a group, isolating them from the main electrical bus until the pilot is ready to power them on.
Context Anchor
You will see the avionics master during cockpit checks, preflight setup, engine start, shutdown, and any that turns airplane electronics on or off.