Have you ever seen a drawing or a storybook with your favorite character on it? Imagine that there were no copies of that drawing or storybook and the only one that existed was with the person who made it. Now imagine that you really want to have your own copy of it. What should you do?
If you are like the people who lived hundreds of years ago, you would probably go find the person who drew it and ask them to make another copy just for you. This is what they used to do with really old books.
When a book was first published a very long time ago, it wasn't like today where it can be printed very quickly and easily. It had to be copied by hand, and there was only one copy of it. This very first copy is called "editio princeps".
So, editio princeps means "first edition" and it refers to the very first copy of an old book that was made by hand a really long time ago.