A firewall is an online security measure to protect your computer from viruses and other malicious attacks.

You can use the internet to communicate with around 4.9 billion people worldwide.

The downside is that everybody also has access to you. This includes hackers and viruses that want to steal your data, take control of your computer or even destroy it.

To stop this from happening, a firewall controls the data flowing between your computer and the internet. A firewall inspects data to make sure it has the right permissions. If it does, it can pass through.

A firewall works at your computer's ports. When we're talking about computer networking, a port isn't the same as a jack or socket you plug your monitor into. Rather it's a virtual entry point where your computer exchanges information with other networks. 

Every computer has lots of ports, each of which handles different kinds of data. For instance, according to the Internet Measurement Conference (IMC), emails often go to port 25 while webpages go to port 80 — even though they both come through the same internet connection