Okay kiddo, so you know how you have lots of friends in your class at school, right? And imagine if your school decided to let other schools join in too, and everyone could become friends with even more people! That's kind of like what's happening with the European Union.
The European Union is a group of countries over in Europe who have joined forces to work together. And they've decided to let more countries join in, so they can all be a bigger group of friends.
It's kind of like when you play with your toys - you might start with just a couple of dolls or cars, but then your other friends come over and bring their toys too, and suddenly you all have a big group playing together!
Now, the people who run the European Union have some rules about who can join the group. Countries have to show that they have good laws, are treating their people fairly, and are working to make things better for everyone who lives there.
When a country does all those things, they can apply to join the European Union. And if they meet all the rules, then they can become a member, just like all the other countries in the group.
So, as the group grows bigger, there are more people working together to make things better for everyone. And that's a good thing, because when lots of people work together, they can solve bigger problems and have more fun together!