“Repetition of any Mantra or Name of the Lord is known as Japa. Japa is an important Anga of Yoga. It is a spiritual food for the hungry soul. Japa is the rod in the hand of the blind Sadhakas (aspirants) to plod on the road to Realization. Japa is the philosopher’s stone or divine elixir that makes one God-like. In this iron age, practice of Japa alone can give eternal Peace, Bliss and Immortality.
Do the Japa with feeling. Know the meaning of the Mantra. Feel God’s presence in everything and everywhere. Draw closer and nearer to Him when you repeat the Japa. Think He is shining in the chambers of your heart. He is witnessing your repetition of the Mantra as He is the witness of your mind.
A Mantra is divinity encased within a sound-structure. It is divine power or Daivi Sakti manifesting in a sound-body. The Mantra is itself Devata.
The sacred Mantra or the Divine Name is a vital symbol of the Supreme Divinity directly revealed in the innermost depths of divine communion to the sages of Self-realization in the hoary Vedic and Upanishadic times. These symbols are in the nature of unfailing keys to gain access into the transcendental realms of absolute experience.
Sounds are vibrations. They give rise to definite forms. Each sound produces a form in the indivisible world, and combinations of sound create complicated shapes. Repetition of a Mantra has a mysterious power of bringing about the manifestation of the Divinity, just as the splitting of an atom manifests the tremendous forces latent in it. When a particular Mantra appropriated to a particular god is properly recited, the vibrations so set up create in the higher planes a special form which that god ensouls for the time being. The repetition of the Panchakshara Mantra – Om Namo Sivaya – produces the form of Lord Siva. The repetition of Om Namo Narayanaya, the Ashtakshara Mantra of Vishnu, produces the form of Vishnu.
The glory of the Name of God cannot be established through reasoning and intellect. It can be experienced or realized only through devotion, faith and constant repetition of the Name.”
Japa Yoga by Swami Sivananda
http://www.dlshq.org/teachings/japayoga.htm