The command and control link (C2) is the radio link between the command unit and the unmanned aircraft: in one direction it carries the pilot's control inputs, in the other — telemetry about the drone's state. Without a working C2 link the pilot has no control over the drone.
The link's integrity is a safety factor: interference, distance and obstacles degrade it progressively, and a complete loss triggers the failsafe. Good practice means frequency hygiene — avoiding RF-polluted locations — and a link check as part of the pre-flight procedure.