The primary function of a firewall in cybersecurity is to monitor and control incoming and outgoing network traffic based on predetermined security rules. Firewalls act as a barrier between a trusted internal network and untrusted external networks, filtering traffic to prevent unauthorized access. They can be hardware-based or software-based and are essential for protecting sensitive data from malicious attacks. By establishing a secure perimeter, firewalls help organizations mitigate the risk of data breaches, ensuring the integrity and confidentiality of their information systems.