A network device through which network traffic passes, such as (commonly) a router/gateway or (sometimes) a bridge/switch, which can filter or otherwise impose arbitrary restrictions on the traffic. It can therefore be used to present a hurdle for someone sitting on one side of the FireWall and trying to do something unwelcome or malicious to a system on the other side. Commonly, a FireWall is more permissive in one direction than the other, thus yielding an inside-vs-outside-the-FireWall configuration. The actual FireWall can be implemented in software (the usual case with routers) or hardware.
Dividing the network in this manner is both useful and problematic:
A FireWall is an effective and financially efficient time-buying measure that protects the systems you control from falling victim to attacks you didn’t have the chance to learn about yet; not more.