Simply:
Eucharist (Greek:, eucharistia, thanksgiving) is the name given to the Blessed Sacrament of the Altar under its twofold aspect of sacrament and Sacrifice of the Mass. In the Eucharist, Jesus Christ is truly present under the appearances of bread and wine, whether it is viewed as a sacrament or as a sacrifice.
A More Detailed Version:
As Catholics, we firmly believe that the real presence of Christ is in the Holy Eucharist. The Second Vatican Council's Decree on the Ministry and Life of Priests (Presbyterorum Ordinis) asserts, "The other sacraments, and indeed all ecclesiastical ministries and works of the apostolate are bound up with the Eucharist and are directed towards it. For in the most blessed Eucharist is contained the whole spiritual good of the Church, namely Christ Himself, our Pasch and the living bread which gives life to men through His flesh that flesh which is given life and gives life through the Holy Spirit" (#5). For this reason, the Council referred to the Holy Eucharist as the source and summit of the whole Christian life (Lumen Gentium, #11).
Our belief in the Holy Eucharist is rooted in Christ Himself. Recall the beautiful words of our Lord in the Bread of Life Discourse in the Gospel of St. John: "I myself am the living bread come down from heaven. If anyone eats this bread he shall live forever; the bread I will give is my flesh, for the life of the world. Let me solemnly assure you, if you do not eat the flesh of the Son of Man and drink His blood, you have no life in you. He who feeds on my flesh and drinks my blood has life eternal, and I will raise him up on the last day. For my flesh is real food and my blood real drink. The man who feeds on my flesh and drinks my blood remains in me, and I in him. Just as the Father who has life sent me and I have life because of the Father, so the man who feeds on me will have life because of me" (John 6:51, 53-57). Note that none of this language is symbolic-- Jesus meant what He said. Moreover, even when there was grumbling and objections, and even after some disciples abandoned our Lord because of this teaching, Jesus no where said, "Oh please, stop. I really meant this symbolically." Our Lord stood by His teaching.
For your (or your child's) first communion, you will need to: