"So hum" is a sanskrit phrase which means "I am that."
Working with mantra is a way to bring the mind into an empty focus by repeating a phrase repeatedly for several minutes, usually in conjunction with a breath technique.
The "So Hum" mantra come from the Hindu tradition which literally translates to:
so = that, divinity
hum = I am
Here's one way to incorporate it into your yoga practice:
Lay on your back with one hand gently resting just below your belly button and the other hand gently resting just below your ribs. Close your eyes and begin to breathe deeply through your nose. Allow your shoulders to relax away from your ears and feel your lungs expand as you inhale. Continue to breathe deeply and completely.
As you inhale, say the sound "So" to yourself. As you exhale, say the sound "hum" to yourself. Inhale the sound "So," for the entire breath, pausing at the top, and exhale the sound, "hum," for the entire breath. Continue this cycle of inhaling deeply the sound "so," and exhaling the sound "hum," for several minutes.
When you feel you have reached a comfortable end to this practice, complete a final cycle of the So Hum mantra and exhale all of the air from your lungs. Your belly button will contract toward your spine as you empty the lungs. Relax your breathing pattern and remain laying down with your eyes closed for a couple of breaths to absorb the benefits of this practice.