The Eucharist is the source and summit of the Christian life.
The Real Presence: At Mass, the bread and wine are changed into the Body and Blood of Jesus Christ. This is not a chemical or physical change because the outward, sensible appearances of bread and wine remain, but under those appearances of bread and wine, Jesus is truly present in the Eucharist.
The Eucharist as a Sacrifice: The Mass is a sacrifice because it re-presents (makes present) the sacrifice of the Cross, because it is a memorial. At every Mass, we are called to join our lives in union with Christ's gift of Himself to the Father.
As a biblical memorial, the Mass makes present Christ's sacrifice on the Cross in order to apply the fruit of His sacrifice to our lives.
Communion: Jesus loves us so much He desired to come to us in Holy Communion. When we receive the Eucharist, we are receiving the very Body and Blood of Jesus into our souls.
It is important to take time for loving thanksgiving with Jesus after we receive Him in Holy Communion, which nourishes and strengthens our life in Christ. Although all are invited, not all are ready to receive the Eucharist, either because of serious sin that has not been brought to the Sacrament of Reconciliation of for lack of full communion with the Catholic Church.