The state of enlightenment or Buddhahood is expressed or manifested in different forms. The physical person of the Buddha is one form of manifestations of Buddhahood. The state of enlightenment is also manifested in the form of words, sounds, syllable and letters. A mantra basically represents the state of enlightenment in the form of word or sound.
Just as Buddhahood manifests in different physical forms, it also manifests in wide range of sounds, syllables and words. The various mantras are concise formulaic verbal and speech representations of the state of enlightenment. Just as physical forms of the Buddha make the state of enlightenment accessible through the visual and tactile faculties, mantras make the enlightenment accessible through the auditory and visual faculties.
There are different kinds of mantras. The innate mantra of reality is the ineffable nature of sound, which is simultaneously empty and audible. This state is expressed in the form of symbolic mantras, which are the syllables, letters and words, which we can chant and also hear.
Among the symbolic mantras, we have seed syllables such as hriḥ for Avalokiteśvara and dhiḥ for Mañjuśrī, which represents the heart of their mantras. Each Buddha’s enlightenment can be represented in different syllables. The full mantras such as Om mani padme hum for Avalokiteśvara and Oṃ ara patsa nadhi for Mañjuśrī and Om ah huṃ vajra guru padma siddhi huṃ for Guru Rinpoche emerge from the seed syllable.