OM NAMO BHAGAVATE SRI RAMANAYA

This mantra was revealed to Sri T.K. Sundaresa Iyer, an ardent devotee, and subsequently approved by Bhagavan to invoke his presence. It is known as the Dwadasakshara Mantra of Bhagavan, which means that it has twelve letters.

In Hinduism it is common to keep the actual mantra hidden, referring to it as a five or six-lettered mantra without actually telling the mantra. This is done to protect the integrity and purity of the mantra so that the uninitiated may not misuse it. When we read in Suri Nagamma's Letters from Sri Ramanasramam that the Dwadasakshara mantra was being chanted before the Cow Lakshmi during her final days, this is the mantra to which she was referring. The actual meaning is, in brief, as follows:

Om
This is the bija mantra of Lord Shiva, deriving its origin from Soham, which indicates the unity of jiva and Shiva. It also refers to the immanent form of Lord Shiva. It is sometimes called Pranava, which is believed to be the fundamental sound behind all creation.

Namo
Mana in Sanskrit means mind. The mind of jiva goes out towards the world. If the word is reversed, mana becomes nama, which means turning to God or Self within.
Namah is a common ending of most divine names meaning "I turn to you." Thus Namo is grammatically necessitated.

Bhagavate
bhaga. There are six bhagas indicative of Divinity. Only God or His true incarnations can possess them. The attributes are all-around power, dharma, fame, prosperity, knowledge and dispassion.

Sri
Has several meanings, but in this context it means "Gracious.

Ramanaya
One who revels in the Self.

So the meaning of the mantra is "I take refuge in the Gracious Divine Lord Ramana."

This is from THE MAHARSHI News Letter