C2 (command and control) is the infrastructure an attacker uses to communicate with devices it has already infected: from an external server it sends commands, receives stolen data and pushes additional malware. Detecting and blocking those outbound connections cuts off the attacker's control even if the malware is still inside the network.
How TecnetOne handles it: TecnetSOC
command and control, C&C, C2 server, beaconing