OM AH HUM
The first three syllables of the mantra represent the three kayas, as well as the vajra body, speech, and mind of all the buddhas. OM corresponds to the dharmakaya, the nature of Buddha amitabha; AH corresponds to the sambhogakaya, and the Lord of Compassion, avalokitesvara; HUM corresponds to the nirmanakaya, manifesting as the Lotus-born Guru, Padmasambhava.
The vajra (dorje in Tibetan) refers to the diamond, the hardest and most precious of all stones. A diamond can cut through all other substances, yet cannot itself be cut by any of them. This symbolizes the unchanging, non-dual wisdom of the buddhas, which cannot be affected or destroyed by ignorance, but cuts right through all delusions and obscurations. It indicates too that the qualities and activities of the body, speech, and mind of the buddhas can benefit all sentient beings, without hindrance from negative forces. Like a diamond, the vajra is free from all defects. Its indestructible strength comes from the realization of the dharmakaya nature, the nature of Buddha amitabha.
As we have seen, the word" guru" in Sanskrit means "weighty," or "heavy." Just as gold is the heaviest and most precious of metals, the guru is the most weighty and most precious of all beings, because of his inconceivable and flawless qualities. Here the guru corresponds, on the sambhogakaya level, to Avalokitesvara, who is endowed with the seven branches of union.
Padma, meaning "lotus" in Sanskrit, indicates the padma family from the five buddha-families. These five families - buddha, vajra, ratna, padma, and karma-are represented by the five buddhas: Vairocana, Akshobhya, Ratnasambhava, Amitabha, and Amoghasiddhi, respectively. Guru Rinpoche is the nirmanakaya emanation of Amitabha, who corresponds to the lotus family and the speech aspect of the buddhas. It is stated in the slltras that simply by uttering the name of Buddha Amitabha, you will be reborn in Sukhavati, the Paradise of Great Bliss, never to be reborn again in lower realms. In the same way, reciting the name of the Lotus-born Guru will bring us every kind of realization. The incomparable qualities of the six-syllable mantra, the Mani, are also described in all the scriptures as being able to bring us to the realization of the bodhisattva levels, or bhumis. The Mani mantra of Avalokitesvara is the sambhogakaya aspect of the Vajra Guru mantra, and also corresponds to the great Vairocana Buddha. This buddha, who is the size of the whole universe, holds a begging bowl in his two hands in the mudra of equanimity. It is said that within this begging bow1 is a lotus with twenty-five rows of petals arranged one upon the other. These rows correspond to the various aspects of the body, speech, mind, qualities, and activity of the buddhas. For example, the body alone has five subdivisions: body-body, body-speech, body-mind, body-qualities, and body, -activity. The present nirmanakaya paradise of the Buddha Sakyamuni is said to rest at the level of the heart, and corresponds to the row of the mind-mind subdivision, this being the reason why in this paradise the precious teachings of the Secret Mantrayana - the Vajrayana - could be taught and spread.
The word siddhi means "true accomplishment." By remembering and praying to the body, speech, mind, qualities, and activity of Guru Rinpoche, both ordinary and supreme accomplishments will be ours. Ordinary accomplishments include freedom from sickness and endowments such as wealth and prosperity; the supreme accomplishment is to attain the complete realization of Guru Rinpoche himself.
Reciting the syllable HUM is like requesting or invoking the guru to come and to bless us with all the siddhis, ordinary and supreme. Our master, Guru Rinpoche, and the mantra are inseparable. So when we utter the name of the guru by reciting the mantra, it's as if we are calling out repeatedly to someone who simply cannot fail to reply. The guru cannot but turn his compassion towards us, and so, if we pray one-pointedly like this, there is absolutely no doubt that Guru Rinpoche will come at once to grant us his blessings. When rain falls on the earth, it falls evenly everywhere, but only the good seeds will germinate, not the rotten ones. In the same manner, the compassion of Guru Rinpoche is unbiased; it is directed universally to all beings, and yet his blessings will be swifter for those who have devotion and faith.
– Dilgo Khyentse Rinpoche - Guru Yoga