Nightclubs and Firewalls
Technical jargon aside, here is a brief explanation that will leave you with a good understanding of what firewalls are and how they work.
To be honest, a computer is very similar to a nightclub. There's always blaring music that some people are dancing to, while others come and go, buy drinks and talk to each other.
Now instead of humans, imagine pieces of information that go in and out of your computer. These pieces of information are sometimes used in programs (dancing), and sometimes they're used to communicate with your computer (buying drinks and talking).
Like nightclubs, you want to make sure that you only let in the right type of people (or information) to access your computer. For instance, you wouldn't want a person carrying a loaded machine gun come into your club.
To meet this goal, discotheques hire doormen and computers use firewalls. For all intensive purposes, they're the same thing.
Much more serious firewalls are used for larger businesses, such as casinos, which employ hardened computer professionals to tweak their firewalls to perfection, so that only truly desired packets of information can come through, leaving your financial and personal details safe and secure within their systems.