Vijnanamaya kosha [wisdom -apparent- body] is the wisdom sheath covering the ‘atma’ or pure consciousness.
Jnana means knowledge but Vijnana means specialized knowledge. This knowledge is not limited to mere information about various subjects like scientific, physical, social, commercial, religious etc. Being well read and informed is useful to progress in the material world but to progress on the spiritual path it is of no use unless coupled with wisdom of self.
Self awareness or ‘Atma Jnana’ cannot be acquired by reading or just studying different subjects or even scriptures. It is deeply rooted in the understanding of the self and knowing that one is not just a body with breath or even the mind but pure consciousness which remains eternal.
Most of us are just caught up in the first three koshas or layers of existence where we give immense priority to our physical, energetic and mental bodies without really stepping back and being a witness of the self.
Having the wisdom to comprehend that one is not just a person of a certain gender, age, characteristic, profession, intellect etc. but an atma or pure consciousness is indicative of them being in the fourth layer of existence. An Atma Gnani has strong vishwas [faith] in being the atma. He knows that the anna or body is his vehicle, the prana or breath is his weapon and the mind or manas is his slave.
The witness is that aspect of ourselves where we stop identifying with our powerful thoughts and self description, and just witness our mind and ourselves. Thus the Vijnanmaya kosha consists of the intellect alongwith the five organs of knowledge. It is charecterized by being interactive and dependent upon the other sheaths for its existence. Meditative and asanas and meditation balance this kosha.