At Mass, during the Eucharistic prayer, the priest says, "Father, we ask you to make these gifts holy by the power of the Holy Spirit, that they may become the body and blood of our Lord Jesus Christ." The priest says this because at the Mass, the bread (or host) becomes the actual body of Christ, and the wine becomes the actual blood of Christ.
At the Last Supper, Jesus took bread, blessed it, broke it, and declared, "This is my body" (Matt 26:26; Mark 14:22; Luke 22:19). Jesus then took the cup of wine, gave thanks, and declared, "This is the cup of my blood" (Matt 26:27-28; Mark 14:23-24; Luke 22:20). When Jesus said these words, the bread and wine became His body and blood. The Apostles were present at this first consecration (from bread and wine to the body and blood of Christ), and when Jesus said, "Do this in memory of me" (Luke 22:19), the Apostles received the power to consecrate (CCC 1337), just as Jesus had. Through Holy Orders, the Apostles passed this power on to the bishops that succeeded them, and these bishops passed this power along to other priests (CCC 1411). So at every Mass, when the priest says, "This is my body," and "This is . . . my blood", the bread and wine change into the body and blood of Jesus. As St. Paul in his epistle to the Corinthians declares: "Is not the cup of blessing a sharing in the blood of Christ? Is not the bread which we break a sharing in the body of Christ?" (1 Cor 10:16). And Jesus said: "My flesh is real food, and my blood is real drink" (John 6:55). (However, it must be stated that the body and blood of Christ [as well as His soul and divinity; CCC 1374] is contained entirely both in the smallest particle of the host, and the smallest drop in the chalice (CCC 1377).
Now, the Eucharistic bread looks like bread, and the Eucharistic wine looks like wine, even after the consecration of the bread and wine into the body and blood of Christ. Not only does it look the same, but it tastes the same. How can this be if the bread and wine have been completely replaced by the body and blood of Christ, and no bread or wine remains? This is so because of a miracle. If we were able to see the consecrated host and wine as they really were, we would see Christ's flesh and Christ's blood. But God allows the accidents of bread and wine (taste, appearance, etc.) to remain, even after the substance of bread and wine has changed into the body and blood of Christ (CCC 1381). God allows the accidents of bread and wine to remain so that we can consume the body and blood of Christ without the discomfort we would feel if we could taste flesh and blood and feel it in our mouths.
This is difficult to believe, and many of Jesus' listeners said, "How can this man give us His flesh to eat?" (John 6:52). This was such a difficult teaching that even some of His disciples, "when they heard this said, ‘‘‘‘this is a difficult teaching; who can listen to it?'" (John 6:60) and "as a result of this many of His disciples withdrew and would not walk with Him anymore" (John 6:66). And God knew that many people would doubt this truth, so through the ages, He gave us many Eucharistic miracles to prove to us the Real Presence of Christ in the Eucharist. The most famous of these is the miracle of Lanciano. Around the year 700, a priest in Lanciano, Italy, was having doubts about the Real Presence of Christ in the Eucharist. But one day, as he said Mass, the bread and wine turned into the body and blood of Christ in a visible way. The flesh is still in Lanciano, and scientific studies have shown it to be human heart tissue. The blood is also in Lanciano, and it exists in the form of five coagulated pellets. When the pellets are weighed, one weighs the same as two, two the same as five, five the same as one, and so on.
Communion means to receive the Eucharist. Holy Communion is food and drink to nourish the soul. Great graces are given to those who receive the Eucharist worthily. One receives worthily by confessing all mortal sins in the Sacrament of Reconciliation and receiving absolution from the priest before Holy Communion (CCC 1415), because "a man should examine himself first . . . anyone who eats the bread or drinks the cup in an unworthy manner shall be guilty of the body and blood of the Lord" (1 Cor. 11:28-29); by having the intention of receiving the body and blood of Christ when communicating; and by abstaining from food and drink, save for water and medicine, for at least one hour before receiving the Eucharist––––according to the rule found in the Code of Canon Law. Graces come in greater abundance if one is fervant and free from sin, which is best done through the Sacrament of Reconciliation. It is also necessary for one to be Catholic to receive the Eucharist, because receiving Communion is a sign of unity, which is not truly shared between Catholics and members of other Christian communities. Catholics are not allowed to receive Communion outside the Catholic Church either, for this same reason, and for the fact that Protestant Communities do not have a valid priesthood, and therefore, Jesus is not truly present in their Eucharist (CCC 1400). Receiving their Eucharist would amount to professing belief that Jesus is truly present in their Eucharist.
Through Holy Communion, the believer is united more closely to God, receives an outpouring of the sanctifying grace which makes the soul holy and makes the soul a dwelling place for God, is forgiven of his venial sins, and receives the actual graces to avoid sin in the future (CCC 1393-1394). Besides the great graces and blessings God gives to those who receive the Eucharist, God gives them the gift of His presence. When we receive the Eucharist, Jesus is physically present within us for the course of the 15-20 minutes it takes to digest the Eucharist (CCC 1377). Jesus should be adored and praised for who He is, thanked for His goodness, and asked for any intentions, for oneself and for others (petition and intercession).
Catholics genuflect in front of the tabernacle upon crossing it or upon entering a pew, in honor of Jesus, who is present in the Blessed Sacrament (the Eucharist). As a sign that Jesus is always present, the lamp perpetually burns in front of the tabernacle (CCC 1378), where the Eucharist is reserved. In respect to Jesus in the Blessed Sacrament, many Catholics visit Jesus in the tabernacle every day, make a Holy Hour of Adoration (spending time in worship, prayer, and spiritual reading in front of the Blessed Sacrament; CCC 1378) during exposition (where the Eucharist is placed in a monstrance and put on display for people to pray to), and attend benediction (a specific layout of songs, prayers, and incensing, in worship of Jesus in the Blessed Sacrament).
We should receive holy Communion every day, because Christ has told us how important the Eucharist is. Jesus 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 on the last day . . . He who eats my flesh and drinks my blood remains in me, and I in him . . . The man who eats this bread shall live forever" (John 6:53-54, 56, 58).