A royal mistress is like a special friend or girlfriend that a king or queen has, but they are not married to them. This person is usually a woman who spends a lot of time with the king or queen and helps them with things. They might even stay with them in the same palace or castle.
However, having a royal mistress was not something that people talked about openly. It was usually kept secret because having a mistress was not considered respectable. So, the king or queen would have to be very careful not to be seen with their mistress in public.
Sometimes, a royal mistress had a lot of influence over the king or queen. They could convince them to do things that they might not have done otherwise. But, it was also risky for the mistress because if the king or queen got tired of them or found a new mistress, they could be kicked out and lose all their power.
Overall, having a royal mistress was a complicated and often secret relationship that some kings and queens had with a special friend who helped them in various ways.