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