The primary function of a network firewall is to monitor and control incoming and outgoing network traffic based on predetermined security rules. Firewalls act as a barrier between trusted internal networks and untrusted external networks, protecting systems from unauthorized access and cyber threats. They can be implemented as hardware devices or software applications. Firewalls analyze data packets and make real-time decisions about whether to allow or block traffic, ensuring that sensitive information remains secure. This critical security measure protects against various attacks, including malware, phishing, and denial-of-service attacks.