What is the dark night of the soul?

Bk 1, Ch 9, #2.
As God sets the soul in this dark night… He allows it not to find attraction or sweetness in anything whatsoever. #4. God transfers to the spirit the good things and the strength of the senses… if it is not immediately conscious of spiritual sweetness and delight, but only of aridity and lack of sweetness, the reason for this is the strangeness of the exchange. #6. If those souls to whom this comes to pass knew how to be quiet at this time… then they would delicately experience this inward refreshment in that ease and freedom from care… it is like the air which, if one would close one’s hand upon it, escapes. #7. In this state of contemplation… it is God Who is now working in the soul. He binds its interior faculties, and allows it not to cling to the understanding, nor to have delight in the will, nor to reason with the memory. #8. God communicates… by pure spirit. From this time forward imagination and fancy can find no support in any meditation.