Okay kiddo, let me explain to you the visa policy of the United States in a simple way you can understand.
Imagine that you want to invite your friend from another country to come and play with you at your house. But before they can come, they need permission from your parents. This permission is like a visa.
The United States is similar to your house, but it's a really big house where many people from all over the world want to come and visit. But just like your parents need to be careful about who they let into their house, the United States also has rules about who they let into the country.
So, people who want to visit the United States need permission to do so. This permission is called a visa. A visa is a special sticker or stamp that goes in a passport, which is like a special book people have to show who they are and where they're from.
There are lots of different kinds of visas for different reasons. Some people come to visit friends like your friend would, some come to work or study, and some come to live in the United States permanently.
But just like your parents wouldn't let a stranger come to your house without getting to know them first, the United States has rules for who can get a visa. People who have done bad things like commit crimes or who have a serious medical condition may not be allowed to get a visa.
Finally, when your friend arrives at your house, your parents will still make sure that they follow the rules and behave well. Similarly, when people arrive in the United States with a visa, they have to follow the rules of the country and not do anything illegal or dangerous.
That's a simplified explanation of the visa policy of the United States.