A firewall is a security system either hardware, software, or a combination of both designed to monitor and control incoming and outgoing network traffic based on predetermined security rules. Its primary function is to create a barrier between a trusted internal network and untrusted external networks, such as the internet, to prevent unauthorized access and cyber threats. Firewalls can filter traffic by IP address, port number, protocol, or specific content, making them a fundamental component in protecting computers, servers, and networks from malicious activity.