What it means
Om, That, Truth. Tat points to what can't be named directly; sat is what is real and endures.
Where it comes from
In the seventeenth chapter of the Gita, Krishna says these three words point to Brahman. He goes on to explain that Om is spoken as sacrifice, charity and austerity begin, tat when acting without wanting the reward, and sat names what is good and true. That's why many people close a piece of work, or even a letter, with Om Tat Sat, offering it up.
How to chant it
At the start or the end of a task, or quietly in meditation.
When: At the beginning or end of anything worthwhile.
Try it now
The japa counter opens with this mantra selected and keeps count for you.
Questions people ask
In verse 23 of chapter seventeen, Shraddhatraya Vibhaga Yoga. The next four verses explain how each word is used.