A firewall is a network security system 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, to prevent unauthorized access and protect against cyber threats. Firewalls can be hardware-based or software-based and work by examining data packets and determining whether to allow or block them based on established security protocols. Firewalls are essential for ensuring the confidentiality, integrity, and availability of network resources and data.