"Soham" means "He I am" or "I am He", "I am Brahman." "Sah" means "He." "Aham" means "I." This is the greatest of all Mantras. This is the Mantra of Paramahamsa Sannyasins. This is an Abheda-Bodha-Vakya which signifies the identity of Jiva or the individual soul and Brahman, the Supreme Self. This Mantra comes in the Isavasya Upanishad: "Sohamasmi."
Soham is only OM. Delete the consonants S and H. You get OM. Soham is modified Pranava or OM. Some like 'Soham' better than 'OM.' Because they find it convenient and easy to associate it or mix it with the breath. Further there is no effort in doing Japa of this Mantra. If you simply concentrate on the breath, if you simply watch the breath, that is quite sufficient.
Meditation on 'Soham' is same as meditation on OM. Some repeat a compound Mantra "Hamsah Soham-Soham Hamsah." Before you practise Soham Dhyana or meditation on "Soham" you must practise the Neti-Neti (not this, not this) doctrine. You must negate or deny the body and the other Koshas by repeating "Naham Idam Sariram"-"Aham Etat Na." "I am not this body, mind or Prana. I am He, I am He-Soham, Soham!"
Repeat this Mantra mentally. You should feel with all your heart and soul that you are the All-pervading, Omnipotent, All-blissful Soul or Brahman. This is important. Then only the full benefits of meditation or Japa of this Mantra will be realised by you. Mere mechanical repetition will not help you much. It has its own benefits. But maximum benefits can only be realised in feeling. Feeling is Self-realisation.
If intellect tries to feel, "I am Brahman, I am Omnipotent" and the Chitta tries to feel, "I am a clerk in the chief court; I am weak; I am helpless. What shall I do for money for my daughter's marriage? I am afraid the judge will fine me," realisation is not possible. You must destroy all wrong Samskaras or impressions, all false imaginations, all weaknesses, all superstitions and all vain fears. Even if you are in the jaws of a tiger, you must powerfully roar-"Soham, Soham, Soham, I am not this body!" Then only you are a real Vedantin. Even if you have nothing to eat, even if you are in the roll of unemployment, you must say with great force and power, "Soham, Soham." You have been spoiled by the mind and by Avidya (ignorance). It is Avidya, it is mind that has brought one to this limitation through identification with this body. Pierce the veil of ignorance. Tear the five sheaths. Remove the curtain of Avidya or nescience, which is a great nuisance. Rest in your own essential Satchidananda Svarupa through the force of Meditation on `Soham' Mantra!
The Jiva or the individual soul is repeating this Mantra 21,600 times within 24 hours. Even during sleep the Soham-repetition goes on by itself. Watch the breath very carefully and you will know this. When you inhale the breath, the sound 'So' is produced. When you exhale, 'Ham' is produced. This is termed Ajapa-Mantra because this is done without moving the lips along with the breath. Repeat 'So' mentally when you inhale, 'Ham' mentally when you exhale. Practise this for two hours in the morning and evening. If you can repeat for 10 hours, it is all the more good. You must practise meditation for full 24 hours when you advance on the path. Study Hamsa Upanishad. You will find real rest during meditation. There is no necessity for sleep.
The repetition of the compound Mantra "Hamsah Soham -Soham Hamsah," creates a deeper impression. The late famous Sri Sheshadri Swami of Tiruvannamalai used to repeat this compound Mantra. When he moved alone in the streets and bazaar he repeated this Mantra. He meditated on this compound Mantra. We say 'God is Love-Love is God.' Similarly repetition of 'Hamsah Sham-Soham Hamsah,' gives more force. This intensifies the force of the Mantra. The aspirant gets more internal strength from the Atman or the Soul-force. His conviction grows stronger. This sort of repetition corresponds to the repetition of the Mahavakya, "Aham Brahma Asmi. Brahmaivahamasmi-I am Brahman. Brahman am I."
"Anal Haq" of the Sufi-Fakirs corresponds to Soham of the Paramahamsa-Sannyasins. Guru Nanak highly extols this Mantra.
Your duration of life is counted really by so many 'Soham' breaths in the light of Yoga. It is not really made up of so many years. By the practice of Pranayama, you save the Soham-breaths and thus can prolong your life.
In the beginning of your practice, simply watch the breath every now and then. Meditate for one hour in a closed room on Soham with Bhava and meaning. You can watch the breath with silent Soham-repetition while sitting, standing, eating, talking, bathing, etc. This is an easy method of concentration.
Soham is the breath of life. OM is the Soul of breath. Eliminate 'Ha' and substitute 'I.' Soham will become 'So I am.' If you concentrate on the breath you will notice that the breath is gradually becoming very, very slow when concentration becomes deep. Gradually the repetition of Soham will drop when there is deep concentration. The mind will become very calm. You will enjoy. Ultimately you will become one with the Supreme Self.