OK kiddo, do you remember when we were waiting in line at the amusement park for that really cool ride? Well, there are some people who think they are more important than others and they don't want to wait in line like everyone else. So, instead of waiting their turn like they are supposed to, they try to sneak in at the front of the line.
Now, you know that's not fair, right? You and everybody else took the time to wait their turn, so it would be really mean if someone just cut in front of you. It's like when we're playing a game together and someone tries to cheat. That's not how the game is supposed to be played and it's not fair to everyone else.
So that's what 'cutting in line' means: when someone tries to skip ahead of others who were waiting their turn. It's a bad thing to do because it's not fair to the other people who were already waiting. We should always respect other people's time and follow the rules of waiting your turn.