The Immaculate Conception refers to the condition that the Blessed Virgin Mary was free from Original Sin from the very moment of her conception in the womb of her mother, Saint Anne. We celebrate the Nativity of the Blessed Virgin Mary on September 8; nine months before is December 8, the Feast of the Immaculate Conception.
Fr. John Hardon, S.J., in his Modern Catholic Dictionary, notes that "Neither the Greek nor Latin Fathers explicitly taught the Immaculate Conception, but they professed it implicitly." It would take many centuries, though, for the Catholic Church to recognize the Immaculate Conception as a doctrine, and many more before Pope Pius IX, on December 8, 1854, would declare it a dogma.
In the Apostolic Constitution Ineffabilis Deus, Pope Pius IX wrote that "We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful."
As Father Hardon further writes, the Blessed Virgin's "freedom from sin was an unmerited gift of God or special grace, and an exception to the law, or privilege, which no other created person has received."
Another misconception people have is that Mary's Immaculate Conception was necessary to ensure that Original Sin would not be passed on to Christ. This has never been a part of the teaching on the Immaculate Conception; rather, the Immaculate Conception represents Christ's saving grace operating in Mary in anticipation of His redemption of man and in God's foreknowledge of Mary's acceptance of His Will for her.
In other words, the Immaculate Conception was not a precondition for Christ's act of redemption but the result of it. It is the concrete expression of God's love for Mary, who gave herself fully, completely, and without hesitation to His service.
For more on the development of the dogma of the Immaculate Conception, see the Feast of the Immaculate Conception.