There’s a popular idea that it refers to Jesus’ conception by the Virgin Mary.
Instead, it refers to the special way in which the Virgin Mary herself was conceived.
This conception was not virginal. (That is, she had a human father as well as a human mother.) But it was special and unique in another way…
2. What is the Immaculate Conception?
The Catechism of the Catholic Church explains it this way:
490 To become the mother of the Saviour, Mary “was enriched by God with gifts appropriate to such a role.” The angel Gabriel at the moment of the annunciation salutes her as “full of grace”. In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God’s grace.
491 Through the centuries the Church has become ever more aware that Mary, “full of grace” through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race, preserved immune from all stain of original sin.
3. Does this mean Mary never sinned?
Yes. Because of the way redemption was applied to Mary at the moment of her conception, she not only was protected from contracting original sin but also personal sin. The Catechism explains:
493 The Fathers of the Eastern tradition call the Mother of God “the All-Holy” (Panagia), and celebrate her as “free from any stain of sin, as though fashioned by the Holy Spirit and formed as a new creature”. By the grace of God Mary remained free of every personal sin her whole life long. “Let it be done to me according to your word. . .”
4. Does this mean Mary didn’t need Jesus to die on the Cross for her?
No. What we’ve already quoted states that Mary was immaculately conceived as part of her being “full of grace” and thus “redeemed from the moment of her conception” by “a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race.”
The Catechism goes on to state:
492 The “splendour of an entirely unique holiness” by which Mary is “enriched from the first instant of her conception” comes wholly from Christ: she is “redeemed, in a more exalted fashion, by reason of the merits of her Son”. The Father blessed Mary more than any other created person “in Christ with every spiritual blessing in the heavenly places” and chose her “in Christ before the foundation of the world, to be holy and blameless before him in love”.
508 From among the descendants of Eve, God chose the Virgin Mary to be the mother of his Son. “Full of grace”, Mary is “the most excellent fruit of redemption” (SC 103): from the first instant of her conception, she was totally preserved from the stain of original sin and she remained pure from all personal sin throughout her life.
Source: National Catholic Registry