What is "om namo bhagavate vasudevaya"?

"Om namo bhagavate vasudevaya is known as the dvadasaksara-mantra ("mantra of twelve syllables"). This mantra is a vishnu-mantra, for by practicing the chanting of this mantra one is elevated to the Vishnuloka (Vishnu is the four-handed form of Krishna. Vishnuloka is where He lives in the spiritual world). It means:

"O my Lord, the all-pervading Personality of Godhead, I offer my respectful obeisances unto You."

In all of ISKCON's temples this mantra is chanted 3 times before beginning to speak on the Srimad-Bhagavatam.

I hope this is helpful.
Sincerely,
Laxmimoni dasi