Hey there kiddo! The law of majestas is a rule that was made a long, long time ago. It's kind of like a rule that says you can't say bad things about the king or queen of your country. It's also sometimes called the crime of "lese-majesty" which means hurting or offending the majesty, or the king or queen.
So, imagine you're at school and someone says something mean about the teacher. That hurts the teacher's feelings and could get the person in trouble. The law of majestas is like that, but it's for the king or queen.
Saying bad things about the king or queen was seen as really bad, because they were the person in charge and everyone had to listen to them. The law was made to protect the king or queen from being insulted or disrespected, especially by their own people.
If someone broke this law, they could get in big trouble, like being put in jail or even being executed. But nowadays, many countries have gotten rid of the law because people believe in freedom of speech, which means everyone is allowed to say what they want, even if it's critical of their leaders.
So, that's the law of majestas, basically a rule that says you can't say mean things about the king or queen of your country.