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 traffic, it helps protect systems from unauthorized access and cyber threats. Firewalls can be hardware-based, software-based, or a combination of both. They can block or allow data packets based on specific criteria, such as IP addresses, protocols, and ports. This functionality is essential for maintaining the integrity and security of sensitive information within a network.