From the Catechism of the Catholic Church, Simplified
Making the Events Present (1084-1085)
Christ, seated at the Father's right hand, pours out his Spirit through the sacraments (which he instituted) and makes present the grace they signify.
In the liturgy, Christ principally makes present his own death and Resurrection. Jesus predicted these events and then lived them out. These events are unique because they do not pass away like other human events. The Paschal events cannot remain only in the past, because Christ's death destroys death. All that he did for us participates in the divine eternity. It transcends all time and is present at all times, drawing everything toward life.
Set in Motion (1086)
Just as Christ was sent by the Father, so he sent his disciples to proclaim that his death and Resurrection had freed us from Satan's power and brought us into the Father's Kingdom. This work of salvation which they preached is set in motion through the sacrifice and sacraments around which the liturgy is centered.
Structured By Liturgy (1087)
When Jesus said "Receive the Holy Spirit" (Jn 20:21), he gave the apostles sanctifying power and made them his sacramental signs. They gave this same Spirit to their successors. This "apostolic succession" structures the Church's liturgical life and is handed on by the sacrament of Holy Orders.
Always Present (1088-1089)
To accomplish this great work of salvation, Christ is always present in all liturgical celebrations. He is present in the Mass in the Eucharistic species, not just in the person of the minister. Christ is the one who baptizes, who speaks in the Scriptures, and is present when the Church prays. "Where two or three are gathered in my name, I am in their midst" (Mt 18:20). In these works, Christ always associates with himself the Church (his Bride) who worships the Father through Christ.
Sharing in Heaven's Liturgy (1090)
In the earthly liturgy, we have a foretaste of the heavenly Jerusalem where Christ is the Minister of the sanctuary. We sing with all the heavenly warriors, venerating the saints, and eagerly awaiting Our Lord Jesus Christ so we can appear with him in glory (Second Vatican Council).
The Spirit and the Church (1091-1092)
In the liturgy, the Spirit is both the teacher of the faith and the artisan of "God's masterpieces," the sacraments. He wants us to live in the risen Christ. In this way, the liturgy is the work of the Spirit and the Church.
In this sacramental dispensation, the Spirit prepares the Church to meet Christ, and unites the Church with Christ's life and mission.