Legal deposit is like a fancy rule that says that when a really important book, newspaper, or other type of publication is made, a special library or organization gets a copy of it for safekeeping. Just like how you make sure to take care of your favorite toy or book, these organizations take care of these important publications so that they can be read and looked at for hundreds of years.
This rule is important because it helps make sure that really important and valuable information is not lost forever. Think of it like a giant treasure chest that people can look through to find out about history, science, art, and more. And when new things are made, like a cool book that just came out, it gets added to the treasure chest so that it can be shared with lots of people. Pretty neat, huh?