The Church calls the Eucharist “the source and summit of the Christian life.” All the other sacraments lead to it and flow from it.
It has many names: Eucharist (thanksgiving), the Lord's Supper, the Breaking of Bread, the Holy Sacrifice, Holy Communion and the Holy Mass.
The Mass is a sacrifice: it makes present the one sacrifice Christ offered on the cross. It is not a new sacrifice but the same one, offered now in an unbloody way. The Mass is also a sacred meal in which Christ feeds us.
At the consecration, the whole substance of the bread and wine is changed into the Body and Blood of Christ, while the appearances of bread and wine remain. The Church calls this transubstantiation.
Jesus is present whole and entire, Body, Blood, Soul and Divinity, under each form and in every part. He remains present in the tabernacle after Mass, which is why we genuflect and why the Church practices Eucharistic adoration.
Holy Communion unites us to Christ, wipes away venial sins, strengthens us against sin, commits us to the poor and builds up the unity of the Church.