The syllables OM AH HUM have outer, inner, and "secret" meanings. At each of these levels, however, OM stands for the body, AH for the speech, and HUM for the mind. They represent the transformative blessings of the body, speech, and mind of all the buddhas. Externally OM purifies all the negative actions committed through your body, AH through your speech, and HUM through your mind. By purifying your body, speech, and mind, OM AH HUM grants the blessing of the body, speech, and mind of the buddhas.
OM is also the essence of form, AH the essence of sound, and HUM the essence of mind. So by reciting this mantra, you are also purifying the environment, as well as yourself and all other beings within it. OM purifies all perceptions, AH all sounds, and HUM the mind, its thoughts and emotions.
VAJRA is compared to the diamond, the strongest and most precious of stones. Just as a diamond can cut through anything but is itself completely indestructible, so the unchanging, non-dual wisdom of the buddhas can never be harmed or destroyed by ignorance, and can cut through all delusion and obscurations. The qualities and activities of the body, speech, and wisdom mind of the buddhas are able to benefit beings with the piercing, unhindered power of the diamond. And like a diamond, the Vajra is free of defects; its brilliant strength comes from the realization of the Dharmakaya nature of reality, the nature of the Buddha Amitabha. SIDDHI means "real accomplishment," "attainment," "blessing," and "realization." There are two kinds of siddhis: ordinary and supreme. Through receiving the blessing of ordinary siddhis, all obstacles in our lives, such as ill-health, are removed, all our good aspirations are fulfilled, benefits like wealth and prosperity and long life accrue to us, and all of life's various circumstances become auspicious and conducive to spiritual practice, and the realization of enlightenment.
The blessing of the supreme siddhi brings about enlightenment itself, the state of complete realization of Padmasambhava, that benefits both ourselves and all other sentient beings. So by remembering and praying to the body, speech, mind, qualities, and activity of Padmasambhava, we will come to attain both ordinary and supreme siddhis.