The Catechism of the Catholic Church statement, “after having attained the age of discretion, each of the faithful is bound by an obligation faithfully to confess serious sins at least once a year” (CCC 1457), includes a footnote reference to the Code of Canon Law:
“After having reached the age of discretion, each member of the faithful is obliged to confess faithfully his or her grave sins at least once a year” (CIC 989).
“Grave sins” here means “mortal sins” so, accordingly, “serious sins” in the Catechism are to be understood as mortal. Keep in mind that for a sin to be mortal, three conditions must be met: grave matter, full knowledge, and complete consent (cf. CCC 1857).
A sin of grave matter which lacks either of the other conditions is not a mortal sin. In such a case the matter is grave but the sin is not. The Catechism explains, “One commits venial sin . . . when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent” (CCC 1862).