Original sin may be taken to mean: (1) the sin that Adam committed; (2) a consequence of this first sin, the hereditary stain with which we are born on account of our origin or descent from Adam. (http://www.newadvent.org/cathen/11312a.htm)

On original sin....the Catechism of the Catholic Church says thus: Paragraph 7. THE FALL 385 God is infinitely good and all his works are good. Yet no one can escape the experience of suffering or the evils in nature which seem to be linked to the limitations proper to creatures: and above all to the question of moral evil. Where does evil come from? "I sought whence evil comes and there was no solution", said St. Augustine,257 and his own painful quest would only be resolved by his conversion to the living God. For "the mystery of lawlessness" is clarified only in the light of the "mystery of our religion".258 The revelation of divine love in Christ manifested at the same time the extent of evil and the superabundance of grace.259 We must therefore approach the question of the origin of evil by fixing the eyes of our faith on him who alone is its conqueror.260 I. WHERE SIN ABOUNDED, GRACE ABOUNDED ALL THE MORE The reality of sin 386 Sin is present in human history; any attempt to ignore it or to give this dark reality other names would be futile. To try to understand what sin is, one must first recognize the profound relation of man to God, for only in this relationship is the evil of sin unmasked in its true identity as humanity's rejection of God and opposition to him, even as it continues to weigh heavy on human life and history. 387 Only the light of divine Revelation clarifies the reality of sin and particularly of the sin committed at mankind's origins. Without the knowledge Revelation gives of God we cannot recognize sin clearly and are tempted to explain it as merely a developmental flaw, a psychological weakness, a mistake, or the necessary consequence of an inadequate social structure, etc. Only in the knowledge of God's plan for man can we grasp that sin is an abuse of the freedom that God gives to created persons so that they are capable of loving him and loving one another. Original sin - an essential truth of the faith 388 With the progress of Revelation, the reality of sin is also illuminated. Although to some extent the People of God in the Old Testament had tried to understand the pathos of the human condition in the light of the history of the fall narrated in Genesis, they could not grasp this story's ultimate meaning, which is revealed only in the light of the death and Resurrection of Jesus Christ.261 We must know Christ as the source of grace in order to know Adam as the source of sin. the Spirit-Paraclete, sent by the risen Christ, came to "convict the world concerning sin",262 by revealing him who is its Redeemer. 389 The doctrine of original sin is, so to speak, the "reverse side" of the Good News that Jesus is the Saviour of all men, that all need salvation and that salvation is offered to all through Christ. the Church, which has the mind of Christ,263 knows very well that we cannot tamper with the revelation of original sin without undermining the mystery of Christ. How to read the account of the fall 390 The account of the fall in Genesis 3 uses figurative language, but affirms a primeval event, a deed that took place at the beginning of the history of man.264 Revelation gives us the certainty of faith that the whole of human history is marked by the original fault freely committed by our first parents.265

Original Sin is the loss of "Original Justice" or "Original Holyness" of a person. Therefore, it would be accurate to say it is something that we don't inherit instead of something we do inherit. It is like there was a great inheritance, but your parents squandered it and now there is nothing for you to inherit. When we are baptized we recieve grace that removes this lacking of grace due to the fall of man, as well as being introduced into the Church.

When Adam and Eve ate the apple...well, when they did that, they made it hard for everyone after them, like with pain during childbirth and everything, they also gave everyone original sin, everyone's born with it, and that's why you get baptized, and if you don't get baptised before you die, you can't go to Heaven becuase of orginal sin

It refers to the decline of man when adam and eve rejected god