Dzogchen is 'The Great Perfection', the apogee of Tibetan Tantric Buddhist meditation accomplishment. It is the Buddha's enlightenment in rainbow light.
Dzogchen is also a path of existential praxis which is the quintessence of all Tantra. But it cannot be taught or learned. Either you get it or you don't. It is the school of Buddhism with greatest affinity to the sanity of twenty-first century mystical aspiration.
A sign of Dzogchen is the Garuda, a mythical bird, Khyung or Kading in Tibetan, an ancient sun-god, the celestial bird sometimes with human face.
'All thoughts vanish into emptiness like the imprint of a bird in the sky'
The exposition of Dzogchen is conventionally expressed in terms of Vision, Meditation and Action. The mystical jargon is highly abstract and abstruse.
One of the great masters of Dzogchen in this century, Kyapje Dunjom Rimpoche, expressed the vision of Dzogchen something like this:
The Dzogchen Vision
'The first thing is vision. Dzogchen vision which sees what really is -- the nature of mind itself. This is the natural state of being, where the mind makes no distinctions and judgments. This state of awareness is called rigpa. Rigpa is naked awareness of the wholistic here and now. We cannot actually express this awareness and there is nothing to compare it to in order to describe it. It is certainly not the ordinary state of emotional confusion and conflicting thoughts, but neither is it nirvanic cessation. This state cannot be produced or developed, and on the other hand it cannot be stopped or extinguished. We can never be free of it and nor can we fall into error in it. It is impossible to say that we actually exist at that moment but we cannot say that we do not exist. This experience is neither of infinity, nor of anything specific.'
'So, to be brief, because the nature of mind, the Great Perfection, rigpa, cannot be established as any specific thing, state, or action, it has the original face of emptiness which makes it pure from the beginning, all pervasive and all-penetrating. Because the unobstructed lustre of Emptiness and the entire gamut of experience whether confused or transcendant are like the sun and its rays, Emptiness is experienced positively as everything and anything whatsoever and it has the intrinsic nature of non-dual awareness of the spontaneously arisen universe of pure quality. For this reason the recognition of the presence of what is, as the primordial natural state of being, the Real Self of the Three Buddha Bodies, intrinsic awareness as the union of light and emptiness, is called the vision of the inconceivable Great Perfection.'
On the Dzogchen Teachings page the nature of Dzogchen is described by various adepts through the ages.