The Holy Spirit prepares for the reception of Christ
The assembly should prepare itself to encounter its Lord and to become "a people well disposed." the preparation of hearts is the joint work of the Holy Spirit and the assembly, especially of its ministers. The grace of the Holy Spirit seeks to awaken faith, conversion of heart, and adherence to the Father's will. These dispositions are the precondition both for the reception of other graces conferred in the celebration itself and the fruits of new life which the celebration is intended to produce afterward.
Source: Catechism of the Catholic Church (Vatican.va)