Netfilter is a framework in the Linux kernel that provides various networking-related functionalities, including packet filtering, network address translation (NAT), and connection tracking. It allows system administrators to set up rules that control the flow of network traffic, enhancing security and performance. The most common user interface for Netfilter is iptables, which enables users to define rules for how packets should be handled. This can include allowing or blocking specific types of traffic, redirecting packets, or logging network activity. Overall, Netfilter plays a crucial role in managing network security on Linux systems.