What it means
Plainly, "I bow to Shiva." Drop the Om and five syllables remain, na, mah, shi, va, ya, which is why it's called the Panchakshara. Shaiva Siddhanta also links the five to the five elements: earth, water, fire, air and space.
Where it comes from
The words come from the Shri Rudram in the Yajurveda, where the line runs namah shivaya cha shivataraya cha. Over the centuries that short phrase became a mantra in its own right. In Tamil Nadu it's Namasivaya, and it turns up again and again in the Tirumurai hymns. On the ghats of Kashi you'll hear it morning and evening. No initiation is required. People simply begin.
How to chant it
Sit before a Shivling or a picture of Shiva if you have one; a quiet corner works just as well. A rudraksha mala is customary. Don't rush the syllables. Give each one its full sound. If you're offering water or bilva leaves, let the chanting keep pace with the offering.
When: Any time of day. Mondays, the pradosh hour at dusk and the night of Maha Shivratri carry special weight.
Try it now
The japa counter opens with this mantra selected and keeps count for you.