The 'Rule of Two' in Star Wars novels is a Sith doctrine. It basically means there are only two Sith at a time, a master and an apprentice. This was a way to keep the Sith hidden and ensure their power grew in secrecy while also fueling their internal power struggle and ambition.
Well, in the Star Wars novels, the Rule of Two is a crucial concept for the Sith. It's about maintaining a balance within the Sith order. The master imparts knowledge to the apprentice, and the apprentice, in turn, is constantly trying to overthrow the master. This cycle of power struggle and knowledge transfer is what the Rule of Two is all about. It also helps in the Sith's long - term plan of overthrowing the Jedi. By having only two Sith at a time, they can operate more covertly and efficiently.