Well, areopagitica is a very big word that means a book that was written a long time ago by a man named John Milton. This book was written to help people understand why it is important to have freedom of speech and not be told what to think or say by the government or by other people who have power.
Imagine if someone said you can only like one color or you can only play with one kind of toy. You might feel upset and like you don't have your own voice. That's how some people may have felt in the past when they weren't allowed to speak their minds. Milton's book was trying to change that and let people know that they have a right to express their ideas and thoughts, even if other people don't agree with them.
So, in short, areopagitica is a book that talks about the importance of having the freedom to express your ideas without being told what to do or say by other people.