What is the function of a command and control server?

A command and control server (C2 or C&C) is a server that is used by attackers to communicate with and control compromised systems, such as bots, zombies, or backdoors. The C2 server can send instructions to the compromised systems, such as executing commands, downloading files, uploading data, or launching attacks. The C2 server can alsoreceive information from the compromised systems, such as system information, keystrokes, screenshots, or credentials. References:

Understanding Cisco Cybersecurity Operations Fundamentals (CBROPS) v1.0, Module 3: Network Intrusion Analysis, Lesson 3.4: Malware

Cisco Certified CyberOps Associate Overview, Exam Topics, 3.4 Compare and contrast types of malware