the sacrament of HOLY EUCHARIST

It is also called Holy Communion or Blessed Sacrament. It is the Sacrament of the Body and Blood, Soul and Divinity of Christ under the appearance of bread and wine. This happens during the 'consecration' at Mass when the bread and wine are changed into the Body and Blood of Christ. Catholics use the term 'Transubstantiation' to refer to that fact. 

This is the soul of the Catholic Church, what Vatican II called the "source and summit" of our Christian life. 

 Before Jesus suffered, he told his disciples that  “Whoever eats my flesh and drinks my blood has eternal life” - John 6:54, and that “The bread that I will give is my flesh for the life of the world.” - John 6:51. In fact the whole of Chapter 6 in the Gospel of John is about the Eucharist. He meant what he was saying.  That teaching was so radical and strange that many of his disciples said: 'Who could accept this?', and walked away. He never called them back like: 'Sorry guys come back, I didn't mean it so. I was just speaking figuratively!' And of course, at the last supper afterwards he told them - lifting the bread: 'Take and eat for this is my body...' And lifting the wine, he said: 'This is my blood...' He changed the bread and wine into his body and blood.  

And he told them: 'Do this in memory of me', which meant he expected them to be ever celebrating this event, this Sacrament, which is what we do as Catholics. 

People may sometimes question why Catholics always receive Holy Communion, why Catholics believe in the 'Real Presence' - which simply means believing that in the Holy Eucharist Jesus is literally and wholly present—Body and Blood, Soul and Divinity—under the appearances of bread and wine. Some people may say: 'Where is it in the Bible?' Such people may like to take a look at 1 Corinthians. 10:16–17, 11:23–29; and John 6:32–71.  This is the third Sacrament of Initiation.