A firewall serves the function of network security by monitoring and controlling incoming and outgoing network traffic based on predetermined security rules. It acts as a barrier between trusted internal networks and untrusted external networks, protecting devices from unauthorized access and cyber threats. Firewalls can filter traffic based on various factors, including source and destination IP addresses, protocols, and ports. They help prevent data breaches and malware infections, ensuring that sensitive information remains secure and that potential threats are detected and blocked promptly.