A stateful firewall is a network firewall that tracks the state of connections, not just individual packets. Instead of judging each packet in isolation, it keeps a small record of sessions such as TCP handshakes, established flows, and expected replies. That lets it allow legitimate return traffic automatically while blocking packets that do not fit an existing conversation.
This matters because many attacks try to bypass simple rule sets with spoofed, fragmented, or out-of-context traffic. Stateful inspection helps defenders reduce noise, enforce cleaner policy, and close off traffic that does not belong to a valid session. In practice, it is common at network borders and between internal zones, where it can support segmentation and limit lateral movement. It is not a substitute for application security, but it is a strong middle layer between basic packet filtering and deeper inspection tools.