This is a day late because I was away for a few days at a funeral. The feast of Mary's Immaculate Conception is my favorite Marian feastday (it's also my birthday.)
It's often confused with the Virgin Birth. The Immaculate Conception means that Mary was conceived by her parents in the natural way, but she was free from original sin. Her soul was endowed with a fullness of sanctifying grace. All of that was in view of her vocation to be the Mother of God. The Virgin Birth, of course, refers to the fact that Mary conceived Jesus in a virginal way, through the power of the Holy Spirit and not by any man.
All of us were maculately conceived--that is, in the state of original sin (from "macula," the Latin word for sin.) Mary, instead, was the only human being conceived immaculately--without original sin. ( besides Jesus, of course, who also did not have original sin). But through baptism and the life of grace, Jesus has joined us to himself and sanctifies us. Our life is a daily struggle with sin. We often fall. But our hope is that at the end of our lives we will die free from sin. Mary Immaculate will obtain that grace for us if we ask her.