Incorrect configuration of firewall policies and third-party VPNs can seriously impact IT security. The document discusses five common firewall configuration mistakes that leave networks vulnerable: 1) misconfigured firewall rules that are as dangerous as having no firewall, 2) firewalls that are not properly maintained and updated, 3) firewall rules that are too open and permissive, 4) firewalls that do not inspect encrypted traffic, and 5) firewalls that are not applied consistently across all network traffic. It then provides an overview of different types of firewalls and how they function to filter network traffic and protect networks from threats.