What it means
"I bow to Bhagavan Vasudeva." Vasudeva is Krishna, son of Vasudeva, and also the one who lives in everything. It has twelve syllables, which is why it's called the Dvadashakshara.
Where it comes from
Most editions of the Srimad Bhagavatam open with these very words, as an invocation before the first verse. In its fourth book, five-year-old Dhruva, stung by his stepmother's words, sets off for the forest, and Narada gives him this mantra on the way. It carried him through his whole tapasya. Perhaps for that reason it's often one of the first mantras taught to children.
How to chant it
On a tulsi mala. The Bhagavata tradition suggests chanting it upanshu: lips moving, no sound escaping.
When: Mornings, Ekadashi, and during a Bhagavat Saptah.
Try it now
The japa counter opens with this mantra selected and keeps count for you.