A firewall is a system designed to prevent unauthorized access to or from a private network and restrict access to the services running on a firewall-enabled server. In other words, firewalls use policies or set of rules that govern the flow of data packets to and from the outside world. This helps you prevent unauthorized Internet users from accessing private networks connected to the Internet.
The Firewall module adds firewall functionality to your Plesk™ control panel. All data packets entering or leaving the local network shall pass through the firewall, which serves as a gate between your network and the Internet. The firewall shall examine each packet and handle it in accordance with specified filtering rules.