A firewall is a security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Its primary purpose is to prevent unauthorized access to or from a private network and protect it from cyber threats such as hacking, malware, and other types of cyber attacks.

Firewalls can be implemented in both hardware and software form, and they operate at different levels of the network stack. For example, network firewalls operate at the network layer and control access based on IP addresses, while application firewalls operate at a higher level and control access based on specific applications or services.

Firewalls use a set of security rules, also known as policies, to determine what type of traffic is allowed to pass through the firewall. These policies are created and managed by network administrators and can be customized to meet the specific security requirements of an organization. For example, policies can be used to block specific types of traffic, such as email attachments, or to allow specific types of traffic, such as web traffic, to pass through the firewall.

In addition to blocking unauthorized access, firewalls can also be used to monitor network traffic for suspicious activity. For example, a firewall can detect and alert administrators to an unusual amount of traffic from a specific IP address, which may indicate a security breach.