Imagine you have a special book that helps you keep all your private information safe. But sometimes, bad people try to steal this book or read it without your permission. That's why there are special rules and guidelines to help protect your book, called a national information system security glossary.
A glossary is like a big dictionary that explains special words and phrases. The national information system security glossary is a fancy name for a special book that has a list of important words and phrases to help people who work in government and other important places keep their information safe.
For example, the word "firewall" is very important. It means a special type of wall that blocks bad people from sneaking into your computer or phone and stealing information. The glossary will explain what a firewall is and how to use it to keep information safe.
The glossary is very important because it helps people who work with important information like secret plans or personal information, to know how to keep it safe. It also helps people to understand what to do if something bad happens, like if someone tries to steal information or hack into a computer system. So, just like our special book that we keep close, the national information system security glossary is there to help protect special information and keep it safe from bad people.