Most of us may have heard the term, and know it’s related to security in some way, but do you really know what a firewall is? Traditionally, firewalls were built to keep danger at bay—they were doors (or walls) to block fire from coming into another area—hence the name firewall. When you’re thinking in terms of your online security, modern-day hardware and software firewalls are similar. They act as barriers to prevent unauthorized access to your personal information.
http://robertsiciliano.com/blog/2013/12/03/what-is-a-firewall/