The precepts, or laws, of the Church are applications of the laws of God. The Lord established the Church to prescribe whatever was necessary to carry out His teachings. Jesus said, “He who hears you, hears me; and he who rejects you, rejects me.” (Luke 10:16) The Church’s precepts then have the authority of Christ behind them. Their obligatory character is meant to guarantee to us the very necessary minimum in prayer, moral effort, and growth in charity.
This precept ensures preparation for the reception of the Eucharist by making a sacramental confession when one is aware of a grave (mortal) sin.
The Church does not intend to say that yearly confession is sufficient for a practicing Catholic. The Church’s concern is that no one goes on indefinitely living in the state of mortal sin.
In the US, the period set aside to fulfill this obligation (the Easter Duty) is anytime between the First Sunday of Lent and Trinity Sunday. But this can be fulfilled at another time for a just cause.
Failure to meet this minimum requirement is a serious sin.