I am starting to explore Luminous Essence: A Guide to the Guhyagarbha Tantra, where in the first section on the "title", says:
All phenomena of saṃsāra and nirvāṇa are inseparable with the enlightened state; they dwell primordially as the eternal wheel of adornment of enlightened body, speech, and mind. Th is is what is meant by the term secret. It is referred to as such because those who are obscured and unfit to receive these teachings do not realize this, although it is genuinely realized by the profound wisdom mind of the victorious ones. It is secret in the sense that the three bodies go unrecognized due to the veils of temporary obscurations, though they are present within. Moreover, the profound views that reveal this presence are not known through one’s own power, nor are they pointed out by another, and, even if they are, they are hard to realize. In this way, they are hidden. Furthermore, unless this special view, meditation, and conduct are concealed from those who are unfit to receive these teachings, the seal of secrecy will be broken and they will be misconstrued. Therefore, it is not taught to such individuals and must be concealed. In this way, it is secret, both in the sense of being unseen and concealed.
(bold added by me)
What are they saying exactly here? Why must it be concealed exactly? What is at risk?
To me it sounds like they are saying, trying to describe the teachings of this Tantra to someone who is not ready or capable of hearing it will just make it even harder for them to understand its meaning, because they will not realize what you are saying and it will harden them in their own stance in disbelief of the teachings. So it is better to just keep it a secret and let them discover it for themselves.
I could be way off, I know very little about Tibetan Buddhism so far, but I would like to explore what they are saying and not sure the full impact of what they mean when they talk about "the secret".