A firewall is a type of network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Its main purpose is to prevent unauthorized access to or from private networks, while allowing authorized communication. Firewalls can be hardware or software-based, and can filter traffic based on a variety of criteria, such as IP address, port number or protocol type. They are an essential component of any comprehensive cybersecurity strategy, as they help protect against a variety of threats, including malware, hackers, and other malicious actors.