Okay kiddo, have you heard of sports like football, basketball or cricket? Sometimes people can do some things in these sports that is not allowed or is unfair. These things are called illegal sports.
Let me explain further. In a game of football, you have to kick the ball into the goal and score points. But if someone deliberately uses their hands to touch the ball instead of their feet, that's not allowed and it's called a handball. It's like cheating because it gives an unfair advantage to that person's team.
Another example is in basketball. Players are not allowed to push or hit other players because it can cause injury. If someone does this, it's called a foul and they get a penalty.
So, illegal sports means breaking the rules of the game, which is not allowed. It's important to follow the rules in sports because it keeps the game fair and safe for everyone.