St. John was a 16th. Century Spanish Mystic, and (with St. Teresa of Avila), co-founder of the Discalced Carmelites.
He is perhaps most associated with the concept of 'The Dark Night of the Soul', which derives from the title of one of his writings. This is basically the idea of a spiritual crisis, or period of spiritual suffering, which ultimately leads us to closer union with God.
Essentially sin and concupiscence cause us to become attached to things which we than find difficult to 'let go'. However, letting them go is essential if we are to reach the Beatific Vision in paradise, as they effect imperfcations in our souls. This process of separation is painful: we don't like giving up familiar things. For most souls, much of this separation will probably occur in Purgatory, but nearly all of us will experience some of it in this earthly life. Some of the great Christian mystics who reported deep experiences (for example Blessed Teresa of Calcutta is reputed by some to have suffered a 'Dark Night' for much of the last 50 years of her life).
When we experience crises of the spirit, be they the traumas that arise as a consequence of the events in our life, or difficulties we experience at times with our faith, we can dispose ourselves to use these positively, so that ultimately our souls will develop and grow closer to Our Lord.
We can also remeber to pray for the Holy Souls in purgatory, to help them on their way through, and also to encourage the practice - we hope there will be someone to pray for us when it is our turn.