Generally speaking, a firewall is a piece of software or a combination of both software and hardware. The purpose of a firewall is to control all incoming and outgoing traffic that comes in and out of your computer. But this explanation draws a very primitive picture about firewalls.

When we speak about traffic and computers, we refer to the exchange of data packets. Data packets are used in order for computers inside a network to exchange information. This is how we exchange information inside a local network or via the Internet. This is also a way for harmful compute viruses to enter your system. What the firewall will do is check what packets are allowed to come in and what packets are allowed to go out.

Why does the firewall have to do those checks? It all has something to do with security. History has shown us is that there is no such thing as a perfect system. Even the most sophisticated computer designs have flaws. In order to prevent these flaws from entering in your system, you should apply a brute-force approach to data-packet management. Your computer should only talk with the computers that you want it to. A firewall will only allow communication with the computers you choose. It is like a phone that will only allow you to talk with some predefined telephone numbers.

Of course, firewalls are configurable, and you can change the settings if you wish but thats another story. The ancient Greeks had a saying: You cant lock your closet and at the same time throw the key inside it. Nothing is absolutely secure; whatever you do, security can and will always be broken if you dont take measures. Thats why firewalls exist. They are like guards that protect your gate, under your special orders.