A scheduled monument is like a really important toy that needs special protection. It's something that is really old and has a lot of history behind it.
Just like how you have to take special care of your favorite toys, the government has to take special care of these scheduled monuments. They do this because they're really important to everyone and the stuff they tell us about our history. So, no one can touch them or make any big changes to them without asking special people first.
Think of it like a big toy museum where the very important toys have extra security guards to keep them safe. The same thing applies to scheduled monuments - they need extra protection to stay safe and preserve their history for everyone to learn from.