Theosis is also known as divinization or deification. At one level, theosis is the state of being united with God, beginning in this life and consummated in the next.
To work toward theosis, we must cooperate with God by uniting our will, our thoughts, and our actions to God's will, thoughts and actions. Thus, theosis is about seeking true union with God. We do this by seeking our ultimate happiness in Christ who shows us the path through penitence and into His will for us.
At another, deeper, level, theosis is the totality of salvation. In other words, theosis assumes that humans from the beginning were made to share in the life of the Holy Trinity. Therefore, through sacrament and prayer we are being helped against 'missing the mark' (various states of unholiness) in order to participate in the mystical life of God.
Thus, in theosis we are restored to our full potential (or a closeness to God which evens surpasses that which was initially experienced by Adam and Eve in the Garden of Eden).