The Immaculate Conception is one of the most Catholic, most mysterious, and most misunderstood Catholic dogmas. The Immaculate Conception has nothing to do with the conceiving of Jesus within the womb of Mary - even though the Gospel reading at Mass that day is the account of the Annunciation, when the Archangel Gabriel told Mary that she was to be the Mother of Jesus. It’s within the text of that Gospel that Catholicism gets the Immaculate Conception.
Catholics believe in the Immaculate Conception, which means that from the first moment of her conception in the womb of her mother, St. Ann, Mary was kept free from original sin by the power of divine grace. So the Immaculate Conception is about Mary’s conception in her mother’s womb. The Incarnation, on the other hand, refers to Jesus’ conception in his mother’s womb.
When the Archangel Gabriel addressed Mary in the first chapter of Luke’s Gospel, he said, “Hail, full of grace.” (kercharit men in Greek, from the root word charis, meaning grace). She was full of grace because of the Immaculate Conception, a divine gift to her from God. She didn’t earn or merit it. He freely gave it to her, so she could provide a worthy, spotless, and pure human nature for Jesus.