A network firewall is a security device that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It acts as a barrier between a trusted internal network and untrusted external networks, such as the Internet. By filtering data packets, it helps prevent unauthorized access and cyber threats. Firewalls can be hardware-based, software-based, or a combination of both. They can also be configured to allow or block specific types of traffic, such as HTTP or FTP. This ensures that only legitimate traffic is permitted, enhancing the overall security of the network.