Illegal immigration is when people move to a new country without following the rules that the country has set for coming in and living there. Countries have rules to decide who can come and stay, just like how your school has rules about who can join and when.
Sometimes, people want to live in a new country because they need a safer place, better jobs, or other reasons. But if they don't have the right papers or permission, that's called illegal immigration. It means they didn't follow the rules to enter or live in that country.
It's important to understand that countries make these rules to keep everyone safe and organized, just like rules keep your classroom safe and happy.