Definition - What does Guru Mantra mean?
A guru mantra is one that a guru gives to a disciple, who then chants the mantra for his/her spiritual growth. Derived from Sanskrit, gu means “darkness”; ru means “remover”; man means “to think”; and tra means “liberate.” The guru, therefore, is a remover of darkness, and the mantra is a means to liberation.
The guru mantra may be a single word or a series of phrases, but often includes the name of a deity. A guru mantra may also be one in which the yogi seeks the assistance of or wants to honor a guru.
Yogapedia explains Guru Mantra
In yoga, chanting a mantra is a practice that yogis use to focus concentration on one thought and/or still the mind in meditation. The guru mantra can be as simple as repeating the name of the ishta devata, or personal god, or it can be a series of bija (seed) sounds or a meaningful expression. For example, the guru might tell the disciple to repeat the primordial sound, Om (or Aum), as a japa for his/her guru mantra.
A guru mantra may also be more complex, such as this tribute to the teacher-guru:
Guru Brahma, Guru Vishnu, Guru devo Maheshwara,
Guru sakshat, param Brahma, tasmai shri guravay namah.
In this mantra, the yogi appeals to the Hindu Trinity - Brahma (force of creation), Vishnu (force of preservation) and devo Maheshwara (force of transformation). It acknowledges the lessons learned through life experiences and honors both the human and spiritual guru, including the guru within.