A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It is designed to protect the network from unauthorized access, malicious attacks, and other security threats. Firewalls can be hardware- or software-based, and can be implemented as a combination of both.
For example, a firewall might be configured to only allow web traffic from certain IP addresses, or to block all incoming traffic from certain countries. It could also be set up to detect and block malicious traffic, such as viruses or malware.