Om is a mystical syllable.
The syllables between Om and the concluding Dhiih are the first syllables of a syllabary called the arapacana because it begins with A RA PA CA and NA. (A syllabary is like an alphabet, but made up of syllables). This syllabary is found in a number of Buddhist texts, including some Perfection of Wisdom (prajñaparamita) texts. Many of the texts in which A RA PA CA NA (and the rest of the sylllabary) appears are not connected with Manjushri, but according to Dr. Conze (in the introduction to The Large Sutra on Perfect Wisdom) “in later literature is is always connected with the Bodhisattva Manjushri.”
The individual syllables A RA PA CA and NA have no conceptual meaning, although they are seen as having symbolic connections with various spiritual qualities.
Here’s the schema laid out in the Large Sutra of Perfect Wisdom (adapted from Conze):
A leads to the insight that the essence of all things is unproduced.
RA leads to the insight that all things are pure and free of defilements.
PA leads to the insight that all dharmas have been “expounded in the ultimate sense.”
CA leads to the insight that the arising and ceasing of things cannot be apprehended because in reality there is no arising or ceasing.
NA leads to the insight that although the names for things change the nature of things behind their names cannot be gained or lost.
These are all important concepts in the Perfection of Wisdom, although to say they are concepts is a bit limiting — really they’re attempts to describe the indescribable nature of reality.
Dhiih is defined as meaning:
thought , (especially) religious thought , reflection , meditation , devotion , prayer; understanding , intelligence , wisdom