Okay kiddo, so imagine you and your family live in a big house with lots of rooms. Now imagine some of your friends from school who live in a different country want to come visit you and play in your house.
However, before your foreign friends can come into your house, they need to follow some rules. They need to ask your parents for permission to come in, right?
Immigration law works the same way. Every country has rules that people from other countries must follow if they want to come in and visit, work or live permanently.
These rules are like the gatekeepers for your house. They help decide who can come in and who cannot.
Sometimes, people can come into a country without a lot of paperwork, like when people from Canada or Mexico want to visit the United States, but other times people need a visa to come visit for a certain amount of time.
A visa is like a special permission slip from the country that says "you're allowed to come in and visit for this long".
If somebody wants to stay in the country for a longer period of time, like to work or live permanently, then they need to fill out even more paperwork and follow more rules.
Just like your parents need to make sure your friends who visit your house are nice and follow your rules, immigration officials need to make sure people who want to live and work in their country will be good citizens and follow their rules too.
That's why the government can say no to some people, even if they really want to come visit, work or live in the country.
That, in a nutshell, is immigration law. It's like the house rules of a country that people from other countries need to follow if they want to visit, work, or live there.