Okay, so you know how when we play games we have a referee who makes sure everyone is playing fair? The public prosecutor in Japan is kind of like a referee but for really serious things that people can get in trouble for, like breaking the law.
Their job is to investigate crimes and decide if there is enough evidence to charge someone. Then they go to court and try to prove that the person did the bad thing they are accused of doing.
But they also have another really important job, which is to help make sure that the laws are fair and working well for everyone. They do this by working with the police and other people who are in charge of making sure people follow the rules.
So basically, the public prosecutor in Japan is like a really important referee who helps keep everyone following the rules and makes sure that people who break the rules are punished fairly.