The Sacrament of Eucharist, also known as "Holy Communion", is the third of the Sacraments of Initiation. Even though we are required to receive communion at least once per year (our Easter Duty), the Church urges us to receive communion frequently (even daily, if possible). It is called a Sacrament of Initiation because, like baptism and confirmation, it brings us into the fullness of our life in Christ.
In Holy Communion, we are eating the True Body and Blood of Jesus Christ, without which "you shall not have life in you" (John 6:53).
Because of the intimate connection of the Sacrament of Holy Communion to our life in Christ, we must be free of any grave or mortal sin before receiving it, as St. Paul explained in 1 Corinthians 11:27-29. Otherwise, as he warns, we receive the Sacrament unworthily, and we "eateth and drinketh damnation" to ourselves.
If we are aware of having committed a mortal sin, we must participate in the Sacrament of Reconciliation first. The Church sees the two Sacraments as connected and urges us, when we can, to join frequent reconciliation with frequent communion.