The primary function of a firewall in network architecture is to serve as a security barrier that monitors and controls incoming and outgoing network traffic. It acts as a filter, allowing legitimate traffic through while blocking unauthorized access or threats. Firewalls can be implemented in various forms, including hardware, software, or a combination of both, and they use predefined security rules to determine which data packets are allowed or denied. This is crucial for protecting internal networks from external threats, including cyberattacks and malware.