Jesus substitutes his obedience for our disobedience
"For as by one man's disobedience many were made sinners, so by one man's obedience many will be made righteous." By his obedience unto death, Jesus accomplished the substitution of the suffering Servant, who "makes himself an offering for sin", when "he bore the sin of many", and who "shall make many to be accounted righteous", for "he shall bear their iniquities". Jesus atoned for our faults and made satisfaction for our sins to the Father.
Source: Catechism of the Catholic Church (Vatican.va)