When adults want to play a game in the bedroom, sometimes they might like to pretend to be characters that are similar to the ones in stories like fairy tales.
In some stories, the prince saves the princess and takes care of her. In the game, sometimes one adult likes to pretend to be the one who takes care of their partner and makes all the decisions. This is called being dominant.
In the game, the other adult likes to pretend to be submissive, which means they let the dominant person make all the decisions and they follow their lead.
This is just a game that adults play and it's important that both people agree to it, are safe, and respect each other's boundaries. It's not something that should be confused with real life relationships.