Okay, imagine you're playing a game with your friends and there's an adult in charge of making sure everyone is playing fair and following the rules. That's basically what the police complaints authority is but for grown-ups.
The police complaints authority is a group of people who make sure that police officers are doing their jobs correctly and treating people fairly. They are like the referee in a game, only they are watching to make sure no one is getting hurt and that the rules are being followed.
If someone feels like a police officer did something wrong or treated them unfairly, they can make a complaint to the police complaints authority. The authority will then investigate the complaint and decide if the police officer did something wrong or not.
The authority is there to make sure that everyone gets treated fairly, and that police officers are doing their jobs the way they're supposed to. Just like how an adult in a game makes sure everyone is playing by the rules, the police complaints authority makes sure that police officers are following the rules too.