A copybook is like a special book that you use at school to practice writing letters and words neatly. It's kind of like a workbook, but instead of doing math problems or answering questions, you focus on writing.
Inside the copybook, there are usually lines that help you know where to write each letter. You might start by practicing each letter of the alphabet on its own, and then move on to writing small words like "cat" or "dog". As you get better, you might write longer sentences or even stories.
The goal of using a copybook is to improve your handwriting and make it easier for others to read what you write. Plus, it can be really satisfying to see how much better you get over time!