Firewall
A device or service that controls which traffic is allowed between networks based on a set of rules.
The firewall sits at the boundary between your network and the internet, permitting what should pass and denying the rest. The default posture that matters is deny by default, allowing only what has been explicitly justified.
Rule sets decay. Temporary rules become permanent, rules reference decommissioned servers, and any-to-any entries survive long after whoever added them has left. Periodic review is what keeps a firewall an actual control rather than a formality.
Where this comes up
Firewall sits inside our networks and infrastructure work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.