Further reading
Vendors explain firewalls:
Cisco: https://www.cisco.com/ c/ en_uk/ products/ security/ firewalls/ what-is-a-firewall.html [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]
Fortinet: https://www.fortinet.com/ resources/ cyberglossary/ firewall.html
Checkpoint: https://www.checkpoint.com/ definitions/ what-is-firewall/
Palo Alto Networks: https://blog.fuelusergroup.org/ from-first-gen-to-next-gen-the-evolution-of-the-firewall