Sanctification (or in its verb form, sanctify) literally means "to set apart for special use or purpose", that is, to make holy or sacred (compare sanctus). Therefore, sanctification refers to the state or process of being set apart, i.e. "made holy", as a vessel, full of the Holy Spirit of God. The concept of sanctification is widespread among religions, including Judaism and especially Christianity. The term can be used to refer to objects which are set apart for special purposes, but the most common use within Christian theology is in reference to the change brought about by God in a believer, begun at the point of salvation and continuing throughout the life of the believer. Many forms of Christianity believe that this process will only be completed in Heaven, but some believe that complete holiness is possible in this life.
In the various branches of Christianity sanctification usually refers to a person becoming holy, with the details differing in different branches.
Sanctification means that self(attitudes, desires, self-will, etc.) is crucified and that you are no longer carnal(in you thoughts, actions, or mind). So, you get filled with the Holy Ghost. This is also known as the second work of grace. In sanctification it is higher level of purging God does because you’re no longer carnal. Also, God will still show you things about yourself, but it should never be carnal.
The Catholic Church upholds the doctrine of sanctification, teaching that:
Sanctifying grace is that grace which confers on our souls a new life, that is, sharing in the life of God. Our reconciliation with God, which the redemption of Christ has merited for us, finds its accomplishments in sanctifying grace. Through this most precious gift we participate in the divine life; we have the right to be called children of God. This grace is the source of all our supernatural merits and bestows upon us the right of eternal glory.
Saint Paul of the Cross stated that "Prayer, good reading, the frequent reception of the sacraments, with the proper dispositions, and particularly the flight of idleness—these are, believe me, the means of sanctifying yourself.