Advocatus is a Latin word that means lawyer. A lawyer is like a helper or a friend who is really good at solving problems when people get into trouble with the law. Lawyers know a lot about the law and can help you understand it better. They can even go to court with you if you need to defend yourself or someone else.
Just like how you have teachers who teach you about different subjects in school, lawyers study and learn about the law and how it works. They can help people who are accused of doing something wrong and make sure that everyone is treated fairly.
So, advocatus is basically just another word for lawyer, someone who helps you understand the law and can support you if you get in trouble.