Guru Mantra

Gurur Bramha Gurur Vishnu
Gurur Devo Maheswaraha
Guru Shakshath Para Bramha
Thasmaisri Gurave Namaha

Gurave Sarvalokanam
Bhishaje Bhavaroginam
Nithye Sarvadhiyanaam
Dhakshina Moorthaye Namahah!

Guru is Lord Brahma, Lord Vishnu and Lord Maheswara
Guru is nothing but the Supreme Brahman devoid of attributes.
That is the reason why we bow to the Guru.