A firewall's primary function in network security is to monitor and control incoming and outgoing network traffic based on predetermined security rules. It serves as a barrier between trusted internal networks and untrusted external networks, protecting systems from attacks, unauthorized access, and various forms of cyber threats. Firewalls can be hardware devices or software applications and are essential for maintaining data integrity and privacy. By filtering traffic, they help establish a secure environment for data transfer, ensuring compliance with security policies and regulations.