Professional ethics is basically the set of rules that people who work certain jobs must follow to be good at their job and to not hurt anyone. These rules help people make good choices and do the right thing, even when it's hard.
Think about playing a game. If everyone follows the same rules, then it's fun and fair for everyone. Professional ethics works the same way - it makes sure everyone who does the same job follows the same rules so that everyone can do their job the best they can and be treated fairly.
Now, let's say you want to become a doctor when you grow up. There are rules that doctors must follow called medical ethics. These rules tell doctors to be honest, to not hurt their patients, to do their best to heal them and to not discriminate against them. The rules also tell doctors to keep their patient's information private, to not take gifts from their patients, and to not cheat or lie.
These rules are important because they help doctors make good decisions even when they might be tempted to do the wrong thing. For example, what if a doctor is offered a lot of money to prescribe a certain drug even though it won't really help the patient? Medical ethics tell that doctor to say no, because the patient's well-being is more important than money.
Different jobs have their own set of professional ethics. But the goal is always the same: to help people do their job well, safely, and fairly for everyone involved.