Imagine you are playing with your friends and you accidentally break one of your mom's favorite vases. Your friend, who saw what happened, promises not to tell your mom. This is like a secret between you and your friend.
Now, imagine that you are a police officer and you see your fellow officer doing something wrong or against the rules. Even though you know it is wrong, you might not tell anyone because you don't want to get your friend in trouble. This is called the "blue wall of silence."
Sometimes police officers feel like they have to protect each other, even when one of them does something wrong. This can make it hard for people to trust the police and can make it difficult for bad behavior to be stopped.
So, just like how it's important to tell the truth and take responsibility for your actions, it's important for police officers to be honest and follow the rules, even if it means telling on their friends.