Can any one explain what does "Vigyan Bhairava Tantra" mean? What does "Vigyan" i.e Science here refer to?
There have been, in India, two main ways of approach to Reality or the Essential Nature of Self,
Vivekaja mārga. (the path of distinction or discrimination). Systems following this mārga are patanjali yoga & Sankara Vedanta.
Yogaja mārga.(the path of union or integration). Saivāgama has adopted the Yogaja mārga in which the goal is not isolation of the Self from Prakrti or Màyà (in case of Sankara Vedanta) but the integration of the individual Self to the Universal Self or Bhairava and the realization of the universe as the expression of His Sakti or spiritual Energy. The ideal of Saivāgama is not the rejection of the universe but its assimilation to its Source. Vijnānabhairav is an excellent exposition of the yogaja mārga.
Literal meaning: Vijnānabhairav consists of two words, vijnāna and bhairava. We have first of all to understand the esoteric significance of Bhairava. Ksemarāja in his Udyota commentary gives a description of the esoteric meaning of Bhairava. The sum and substance of it is that Bhairava is an acrostic word consisting of the letters, bha, ra, and va; bha indicates bharaña or maintenance of the universe; ra indicates ravaña or withdrawal of the universe; va indicates vamana or projection i.e., manifestation of the universe. Thus Bhairava indicates all the three aspects of the Divine. From the mystic point of view Bhairava is the ultimately reality in which there is no distinction of Shiva & Shakti. The svarupa or essential nature of Bhairava is vijnāna or bodha or mahābodha, cit or caitanya the main characteristic of which is svatantrya or absolute freedom revealing itself in icchā, jnāna & kriyā. It is to this vijnānabhairav that the seeker of spiritual life has to be integrated.
The entire manifestation consisting of subject and object is a mere reflection in this vijnāna. Just as a city in a mirror appears as something different from the mirror, though it is nothing different from the mirror, even so the universe though appearing different from vijnāna is nothing different from it. In verses 2 to 6 of vijnānabhairav, the Devi mentions certain well known statements about Bhairava and wants to know His highest state or essential nature. Bhairava categorically rejects the various well known opinions about his highest state and pithily but with luminous clarity states in verses 14 and 15 what His essential nature consists in :
The highest state of Bhairava is free of all notions pertaining to direction, time, nor can that be particularized, by some definite space or designation. In verity that can neither be indicated nor described in words. One can be aware of that only when one is completely free of all thought-constructs. One can have an experience of that bliss in his own inmost Self (when one is completely rid of the ego, and is established in the plenitude of the divine I-consciousness).
That state of Bhairava which is full of the bliss of non-difference from the entire world is alone Bhairavi or Sakti of Bhairava. That state is vijnāna-a state of consciousness which is free of all thought-constructs. This vijnānabhairav is the goal of man.
Content of Vijnānabhairav Tantra- In short - Devi asks - " Who are you" Lord Shiva in reply end up replying the methods to know him. Methods which are 112 in number, all are sufficient to realize that Bhairava, becomes the content of Tantra/Āgama.