This is the sovereign knowledge, the sovereign secret, a supreme purifier. It can be grasped directly, it accords with dharma, it is very easy to practice, and it never runs out.
Explanation
Krishna is describing the teaching he began in the previous verse, the knowledge joined with realization that frees one from what is inauspicious. He calls it the "king of knowledge" (rājavidyā) and "king of secrets" (rājaguhya), meaning the highest and most hidden. It is "in keeping with dharma" (dharmya), that is, with right living, and the verse says it is directly perceivable, easy to practice and imperishable (avyaya). Commentators differ on whether the "knowledge" points chiefly to devotion to God or to the non-dual self, and the verse itself leaves this open.