In plain words
The Guru is Brahmā, the Guru is Viṣṇu, the Guru is the god Maheśvara; the Guru is the very Parabrahman: to that holy Guru, salutations.
Brahmā: he is the Guru. Who is this Brahmā? Brahmā is the Creator, the one who rules everything. He himself is creation; he himself is the Creator. He himself pervades this whole vast world! ‘Sarvaṃ brahmamayaṃ jagat.’ So it becomes clear that the one who has taken this form of everything is the true Guru.
‘Gururviṣṇuḥ’: who is Viṣṇu? The one who bears the conch, the discus and the mace? No, no. ‘Viṣṇu’ means the one who has the quality of pervading. He Himself is the doer, He Himself the deed! The whole world is a form of action. God Himself is its doer. The Consciousness that is the cause of every deed and action is itself the form of God. He Himself is the cause, He Himself the doer, He Himself the duty. The one who has become everything Himself is Viṣṇu. ‘Viśvaṃ viṣṇusvarūpaṃ.’ Viṣṇu means the very form of the universe! He is the Guru.
The Guru is the one who roots out the darkness of Ajñāna
What does Guru mean? Is the Guru the one who teaches by reciting mantras? No, no.
The syllable gu is beyond the guṇas; the syllable ru is without form; the syllable gu is darkness, the syllable ru its removal.
The syllable gu means Ajñāna, darkness. What roots out this darkness of Ajñāna? Light! So ‘Guru’ means the very one who roots out the darkness of Ajñāna! The Guru is not one who merely gives a mantra. Nor is the one who teaches some kind of Vedānta the Guru. And none of those we call by these worldly names is a Guru. All these may be teachers. Among these teachers, the one who practises and then teaches others may be an ‘ācārya’. ‘Ācārya’ means the one who shows it by practice. Today there are no ācāryas and no Gurus. ‘A mantra in the ear, money in the hand’: this is not the mark of a Guru.