In the network device what is a firewall?

Computer firewall is a security software which acts as a barrier and protects the system from black-hat intrusion attempts. Firewalls also block communication attempts into or out of your computer; certain types of malware or software bots that attempt to send or receive data messages will be blocked. A firewall program is your computers perimeter fence, designed to keep bad guys out, and to prevent data passing into/out of your computer. Firewalls are configured in software and hardware devices with specific criteria to block or prevent unauthorized access. It has the packet filtering mechanism which checks for the packets that contain data, prevents or blocks it depending upon the rules programmed in it. Reference source: personalfirewall.comodo.com