Okay kiddo, have you ever heard of encyclopedias? They're like really thick books that have a lot of information about lots of different things.
Well, a CD-ROM encyclopedia is like one of those encyclopedias, but it's stored on a disc that you put in your computer. It's kind of like a big DVD, but it's for learning things instead of watching movies.
When you put the CD into your computer, it opens up a program that has all sorts of information about different topics, like science, history, or animals. You can search for whatever you want to learn about and the program will show you articles, pictures, and sometimes even videos about that topic.
The cool thing about CD-ROM encyclopedias is that you don't have to go to a library to find information - it's all right there on your computer! They were really popular back in the day before the internet became super popular and everything was online.
So, that's it, kiddo. A CD-ROM encyclopedia is a big informational program on a disc that you can put in your computer to learn about lots of different stuff. Cool, right?