What is the mind?
The mind has four aspects:
1) Manas, the ability to think; to conceptualize experience
2) Chitta - the mind stuff itself; it can be likened to the ocean and vritta its waves/whirlpool - it is also the content/storage of information as memory
3) Ahamkara - the ego as the mentally generated I-entity [the false 'I']
4) Buddhi - the higher mind; the ability to discriminate; the will
Thus, the definition of yoga given in Patanjali's Yoga Sutras as: yogas chitta vritta nirodhah is understood as 'seeing through the mind' rather than 'seeing with the mind'
Two things are necessary for this quality of seeing as Buddhi, willful discrimination:
1) the mind must be silent, like a perfectly calm ocean;
2) the mind must be absolutely free of content, innocent; like the ocean water being perfectly clean/clear.
What is meant by a yoga asana/pose?
Asana literally means "seat" - it means to "take your seat" and when this is translated as the word "pose" it means the biblical direction to "be still and know". In other words "meditation".
The understanding that flowers out of this practice is the other biblical expression, "I am that I am" or said another way, "I am the knowing by which I know that I am." This is the experiential understanding of Nonduality.