Firewalls are the “bouncers” of network infrastructure ensuring that any unauthorized traffic is blocked from entering a given network or device. Specific parameters can be put in place to define what is allowed and what is not, traffic from defined ip ranges, locations, types of devices even types or traffic. There are various types of firewalls as well including, stateful, packet-filtering (the most basic type) also proxy. some functioning on different levels of the OSI model.
It is important that firewalls be set up correctly as a breach could potentially be devastating to a company and its data, leading to stolen data or attacks.