Mantra Bhairava

The fourth state of the observer is called Shuddhavidya and its observer is called mantra pramatri. In this state, the observer is always aware with svatantraya. All the malas have been removed and its observer observes only the state of his own self, his own Real nature, full of consciousness, full of bliss, full of independent will, full of knowledge and full of action. Hence, this state, though it is not a stable state, is the real state of Shiva. The mantra of this state is aham aham, idam idam.

The meaning of the first section of this mantra, aham aham, is that in this state the yogi experiences that he is the reality, the Real nature of self, the truth of this whole Universe. 

The meaning of the second section of this mantra, idam idam, tells us, on the other hand, that he also experiences that this universe is false, that is unreal. 

The experience comes and goes. Sometimes he experiences this state and sometimes he does not experience it. Sometimes he experiences only aham aham. Sometimes when his consciousness is a little damaged, he experiences only idam idam

Therefore, his reality of self remains unstable and uncertain.

- Kashmir Shaivism, The Secret Supreme., Revealed by Swami Lakshmanjoo

Mantra Bhairava

Here, knowledge means the supreme knowledge of oneness. It is, in the real sense, the supreme God who is the formation of the collection of all sounds. It is from this point that all sounds are created and stored. So, in another sense, it is the state of God consciousness that is one with the universe and filled with supreme I-consciousness - Shiva Sutras, Swami Lakshmanjoo

This is the essence of all mantras. By the words “essence of all mantras” is not meant the mantras such as om namah shivaya, om namo bhagavate vāsudevāya, etc. These mantras are not mantras in the real sense. Mantra, in the real sense, is that supreme I-consciousness. This is the secret about mantras.

This is also quoted in Śrī Tantrasadbhāva Tantra.

All letters are actually one with mantra and those mantras are one with mother, the energy of lord Shiva, and that mother is one with lord Shiva himself. - Swami Lakshmanjoo