Krishna expands on the previous verse. The person who has mastered the mind finds it to be the most reliable companion. A disciplined mind brings clarity to decisions, peace to daily life, and steadiness in difficult moments.
But for the one who has not gained that mastery, the very same mind turns hostile. It lures toward wrong paths, amplifies greed, manufactures fear. The mind does not sit neutral — it is either working for you or working against you. There is no middle ground.
A horse that has been trained carries you wherever you need to go. An untrained horse throws you off and runs wild. The horse is the same in both cases. The difference is training. Krishna is saying: train your mind, and it becomes your greatest asset.