A firewall is a piece of software that sits on a server or a personal computer to help protect it against hackers and other intruders.

Like a literal wall of fire around a building would do, a firewall creates a buffer zone between the outside world and the server or computer being protected. It inspects everything passing in and out of the computer network and uses rules to identify suspicious behaviour - blocking anything it is not too sure about.

The behaviour of some firewalls is so strict that even legitimate information can be blocked, and in this case certain rules have to be set up to allow that information through.