Packet Filtering Firewalls are security devices that monitor and control incoming and outgoing network traffic based on predetermined security rules. They inspect packets of data and determine whether to allow or block them based on criteria such as source and destination IP addresses, port numbers, and protocols. This process helps protect networks from unauthorized access and potential threats. These firewalls operate at the network layer of the OSI model, making them efficient and fast. However, they do not analyze the content of the packets, which means they may not detect more sophisticated attacks that exploit application-level vulnerabilities.