A Firewall is the technical name for a barrier between your computer and the rest of the Internet. It is like the locks on the doors of your home. Most of your neighbors can probably be trusted not to walk into your home and vandalize it or steal from you. Usually only a small number of your neighbors are untrustworthy. But, if you live in heavily populated area, there are a greater number of dishonest people around.
The Internet is similar except that your immediate neighborhood consists of hundreds of millions of people. Even the small percentage of those people who have a destructive bent is a large number of people.
Novell Client Firewall (NCF) not only locks your computer's doors, it makes your computer invisible on the Internet. Your computer normally lets other Internet users know its address. It is like the address sign of your home or the license plate of your car. Your computer's address is plainly visible. NCF prevents your computer from broadcasting its address unless specifically authorized. Hackers are not just kept out; they cannot even find out that your computer is connected to the Internet.