What is the Immaculate Conception? Is it about Jesus' birth?

A common mix-up: the Immaculate Conception is about Mary being conceived in St. Anne's womb, not about Jesus being conceived in Mary's. (Jesus' conception is the Annunciation and the Virgin Birth.)

The dogma. In 1854 Pope Pius IX defined 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" (Ineffabilis Deus; CCC 491).

Why the Church believes it. The angel greets Mary as kecharitomene, "full of grace" or "having been graced completely" (Lk 1:28), a perfect participle indicating a completed state. Mary is the new Eve (Irenaeus, second century): as Eve was created without sin and said no, Mary was created without sin and said yes. She is the Ark of the New Covenant, and the Ark had to be pure. The Fathers called her "all-holy" (Panagia) from the earliest centuries.

She still needed a Savior. Mary sang, "My spirit rejoices in God my Savior" (Lk 1:47). Bl. John Duns Scotus explained: there are two ways to be saved from a pit, being pulled out or being kept from falling in. Mary was saved the second way, by the same Cross that saves us, applied in advance (CCC 492).

Why it matters. It shows what grace can do with a human being, and what God intends for all of us: complete freedom from sin. The feast is December 8, a holy day of obligation and the patronal feast of the United States.