Raja vidya means "the king of knowledge". If you learn this knowledge, then you become completely freed from the conditioned stage of this material world. Therefore it is called raja. Raja means king, and vidya means knowledge.
The Nectar of Instruction - Upadesamrta is the key to enlightenment for all seekers on the path of spiritual perfection. How to choose a guru, how to practice yoga, even where to live—you'll find it all in this invaluable work originally written in Sanskrit by Rupa Goswami, the greatest spiritual genius of medieval India.