"The Eucharist, the sacrament of our salvation accomplished by Christ on the cross, is also a sacrifice of praise in thanksgiving for the work of creation. In the Eucharistic sacrifice the whole of creation loved by God is presented to the Father through the death and the Resurrection of Christ. Through Christ the Church can offer the sacrifice of praise in thanksgiving for all that God has made good, beautiful, and just in creation and in humanity."
Catechism of the Catholic Church 1359.
The Holy Eucharist is the most important of the seven sacraments because, in this and in no other sacrament, we receive the very body and blood, soul and divinity of Jesus Christ. Innumerable, precious graces come to us through the reception of Holy Communion.
Communion is an intimate encounter with Christ, in which we sacramentally receive Christ into our bodies, that we may be more completely assimilated into his. "The Eucharist builds the Church," as Pope John Paul II said (Redemptor Hominis 20). It deepens unity with the Church, more fully assimilating us into Christ (1 Cor. 12:13; CCC 1396).
The Eucharist also strengthens the individual because in it Jesus himself, the Word made flesh, forgives our venial sins and gives us the strength to resist mortal sin. It is also the very channel of eternal life: Jesus himself.
In John’s gospel, Jesus summarized the reasons for receiving Communion when he said:
"Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. For my flesh is real food, and my blood is real drink. He who eats my flesh and drinks my blood abides in me, and I in him. As the living Father sent me, and I live because of the Father, so he who eats me will live because of me. This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread will live forever" (John 6:53–58).
Because of the gravity of Jesus’ teaching on receiving the Eucharist, the Church encourages Catholics to receive frequent Communion, even daily Communion if possible, and mandates reception of the Eucharist at least once a year during the Easter season. Before going to Communion, however, there are several things one needs to know.
Catholics and Communion
The Church sets out specific guidelines regarding how we should prepare ourselves to receive the Lord’s body and blood in Communion. To receive Communion worthily, you must be in a state of grace, have made a good confession since your last mortal sin, believe in transubstantiation, observe the Eucharistic fast (one hour), and, finally, not be under an ecclesiastical censure such as excommunication.
(Taken from Catholic Answers).