Om is the word denoting God.
It should be recited repeatedly while dwelling mentally on its meaning.
From that practice the consciousness turns inward and the obstacles are overcome.Yogasutra 1:27-29
Once an old rishi and his disciple went on a long pilgrimage to visit holy places and receive the blessings of great saints. Often people gathered around them to listen to the rishi, whose simple and wise words went directly to their hearts. For the rishi was indeed a saint and a great guru, although he was clothed like a beggar and didn't seem to have many disciples. He didn't talk much, and sometimes many days passed by without him saying anything at all, and people often wondered why.
-Silence is power, the rishi once said. What is the use of talking? You learn through silence and not through words.
But sometimes he told stories, and if there were other people around they usually came to listen. The disciple always loved his stories, and often asked him to tell one.
One day, having crossed the whole Indian continent, the rishi and his disciple came to the sea. The disciple had never seen the sea before and was immediately fascinated by the endless water, all the seagulls and above all the waves.
-The waves come and go, come and go, come and go; and every time they just disappear and then come back again. Why is it like that? Has it got anything to do with the sound they make?
- If you want, I can tell you a story about it, began the rishi.
-Yes! said the disciple eagerly.
-Well then, once upon a time, before the sea existed, there was nobody living here except Shiva and Shakti. You know Shiva and Shakti? Shakti is the most beautiful of all goddesses, and Shiva is her husband and lover. They love each other so much that even a short moment of separation is terrible for them. So separation became part of their spiritual practice, their sadhana.
Shiva went away to Mt. Kailash to live as a hermit for a while, and Shakti was not allowed to go with him. She stayed alone, below in the mountains, and felt very sad. Her longing for Shiva was so strong that she didn't know what to do with herself. Everything around her seemed to be boring and purposeless, and she felt very restless.
So Shakti went around to all the gods to ask for advice, but they couldn't help her. When she was most desperate, she even went to Yama, the god of death. But he refused to see her as she was not on his list of expected visitors.
Finally she went to Indra, and told him about her difficulties. He listened, and then said: You will meet Shiva again. In the meantime you must practise sadhana; this will serve two purposes. First, it will help you to progress spiritually, and will decrease the time you have to live without Shiva. You will also feel a certain inner contact with him as he is practising sadhana also. Secondly, your restlessness will disappear. The practice is to meditate on Om, the beginning-less sound. This practice will take you very far on the spiritual path. Just sit down for meditation, close your eyes and repeat Om loudly every time you exhale.
So when the evening came Shakti sat down in padmasana, and started repeating Om loudly. She found so much peace in this simple practice, that it was impossible for her to stop. She sat in deep meditation for days, months and years. Very heavy rains poured down on Shakti, but her meditation was so deep that she didn't notice it at all. In a short time the ground around her was covered with water. Soon the water level had reached her waist. It rained and rained, and still Shakti didn't notice anything. Finally her whole body was covered with water, but her meditation was unbroken. She was still repeating, Om, Om, Om...
And this is the sound of the sea, even today. At the bottom of the ocean, Shakti is still sitting deep in meditation, repeating her mantra. Every time she breathes in, the waves go back; every time she breathes out, they come forward again. So Om, the mystical eternal sound we hear in the ocean, is the voice of God. And so the ocean will go on roaring Om, Om, Om... until Shiva and Shakti are united again. And the sea is the best storyteller in the world. Just listen. You can learn everything from it, for everything is the sound of Om. This is the word which is powerful in silence.
Shiva is consciousness. Shakti is energy, in the lower aspect, matter, and in the higher aspect, prana. The union of consciousness with the lower aspect of energy brings forth creation. Highest, however, is the union between the prana aspect of Shakti and Shiva. This is samadhi. So, in the higher sense, Shiva and Shakti are separated during creation, and every spiritual aspirant tries to unify them.