Okay kiddo, so you know how when you play a game with your friends, sometimes you make up rules and everyone has to promise to follow them? Well, becoming a citizen of a country is like playing a really big game where everyone has to follow lots of really important rules.
When someone wants to become a citizen of a country, they have to take something called an "oath of citizenship". This is like making a really serious promise to follow all the rules of the country and be a good citizen.
The oath of citizenship usually includes promising to be loyal to the country, to obey its laws, to defend it from harm, and to respect other citizens and their cultures. It's a way of showing that you really want to be part of the country and that you promise to do your best to be a good citizen.
So, just like when you promise to play a game by the rules with your friends, taking the oath of citizenship is a way of promising to be a good citizen and follow the rules of your new home sweet home.