Catholicism professes that during the Consecration, a miracle occurs — the priest consecrates the bread and wine: Just as Jesus did at the Last Supper, the priest takes the bread in the form of a Host and says, "This is My body." Then he elevates the Host for the congregation to see, bells are rung, and he genuflects. Then he takes the chalice (cup) of wine, saying "This is the cup of My blood," elevates the chalice, and genuflects. Now it's the body and blood of Christ — it still looks, feels and tastes like bread and wine, but it's not. This change of bread and wine into the real Body and Blood of Christ is called transubstantiation.
The Bible says that God created merely by speaking: "God said, 'Let there be light' and there was light" (Genesis 1:3). Likewise, by merely speaking the words of Christ over the bread and wine during Holy Communion, the priest changes them into the body and blood, soul and divinity of Christ through the authority given to him by the Sacrament of Holy Orders. Only an ordained priest has the authority to say Mass and consecrate the bread and wine.
Catholics kneel before the consecrated Host — the Eucharist — because it's not a piece of bread anymore — it truly is Christ. If the Holy Eucharist were just a symbol — such as bread and wine — then kneeling down and adoring it would be considered idolatry, but the Catholic Church has staunchly asserted for 2,000 years that the Holy Eucharist isn't a symbol. The Holy Eucharist is his body and blood. Therefore, the Holy Eucharist is Christ himself present in the consecrated Host...Catholicism for Dummies by John Trigilio