Smashing Magazine has a well-written article on firewalls: A Comprehensive Guide To Firewalls. The writing must be comprehensible to anyone even if he’s not aware of the technical jargon involved. The writer does the trick by some similes that make sense.
A computer is like a big housing complex. Every computer on the Internet has a numerical address, known as an IP address. At each address are two very large blocks of apartments. Each block contains 65,535 individual apartments. The vast majority of them are empty, but a few, especially the lower ones, have very communicative residents. All communication is by mail.
If you are lost about firewalls or if your knowledge is limited, give it a read.