Okay kiddo, do you know what a cupbearer is? It's someone who carries a cup for an important person, like a king or queen. And a master is someone who is really good at something and in charge of others who do that same thing too.
So a master of the cupbearers is someone who is really good at carrying cups for important people, and is in charge of other people who carry cups too. They make sure everything is done properly and that the important person they work for has their cup when they need it. It's kind of like being the captain of a team.