When a firewall checks if data can enter your network, it'll read a message that comes with it called metadata. This will list a string of numbers indicating where the data has come from (known as the source address), where it's going (the 'destination address' aka your PC) and over which port.

Whether the data has permission will all depend on a set of rules known as a protocol. These rules can be set to prevent you — or any other particular user – from uploading certain files to the internet.

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...