Much like a firewall in the physical world, a firewall in computing terms is software or hardware that helps prevent danger (malicious users, hackers, viruses) from reaching your computer through the internet.
Firewalls monitor traffic and connections coming in and out of your computer. By preventing connections or information from users that could be dangerous, you significantly reduce the chance of your computer being hacked or infected.
Do I need antivirus software and a firewall?
Yes, they perform related but separate tasks to help secure your computer from online threats. You could think of firewalls as the guards at the gate preventing danger from getting in and antivirus software as the ones inside finding anyone that slipped through and throwing them back out.
How do I get one?
Windows XP SP2, Vista, 7, 8, and higher come with a firewall that is installed and enabled by default, so if you use one of these systems you are likely already protected. Otherwise most popular antivirus or cyber-security suites provide a firewall as part of their package, although some only include this option in their paid tiers. If you don’t have a firewall already, check whether the anti-virus package you are using has an option to upgrade to a version that includes a firewall.