A firewall is a device or program that controls the flow of network traffic between networks or hosts with different security needs, allowing or blocking traffic according to a defined security policy.
What it means
NIST defines a firewall as a gateway or control point that limits access between networks based on a security policy. In practice it acts as a filtered doorway: it lets approved traffic through and blocks unauthorized connections. Firewalls are a foundational concept in networking, systems administration, and security work.