A firewall is a network security system, implemented in either hardware or software, that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It establishes a protective barrier between a trusted internal network and an untrusted external network, such as the Internet. By analyzing data packets and determining whether they should be allowed through or blocked, a firewall serves as a first line of defense in cybersecurity, preventing unauthorized access, malware, and other cyber threats from infiltrating a private network while permitting legitimate communications.