A Firewall basically acts as a regulatory and monitoring body between computer network and different zones of trust (a zone of trust is simply a separate area of coverage - i.e. home network is a zone and the internet is another zone).

The basic idea of a firewall is to regulate the traffic and provide controlled flow of data between different zones of trust (i.e. between your computer and the Internet for example). A network administrator or needs to understand the basic understanding of network protocols and computer security while using a firewall failing which may result in a firewall acting as a useless security tool. Fortunately, a lot of products on the market allow you to set up firewalls on your own PC (aka "Personal Firewalls) or on your home network without much of the hassle - some of them just need to be plugged in or installed and you can leave it at that.