Okay kiddo, so a harmonised monitoring scheme is when lots of people work together to check that certain things are all the same.
For example, imagine there are lots of different car companies making cars. They all want their cars to be safe, so they have to follow some rules called regulations. But, each company might follow the rules in slightly different ways. To make sure that all the cars are really safe, a group of people might work together to check that all the car companies are following the regulations in the same way. This is called a harmonised monitoring scheme.
The people who work together might make a list of things that the car companies have to do to follow the regulations. Then they might go check each car company's cars to make sure they are all doing those things correctly. If one company isn't doing something right, they'll tell them how to fix it. This way, all the cars will be equally safe and follow the same rules.
A harmonised monitoring scheme can be used for lots of different things, like making sure food is safe to eat or making sure buildings are strong enough to stay standing. It just means that lots of people are working together to check that things are all the same and follow the rules.
Did that help you understand, little one?