In the first chapter of the Gospel of Luke we read of the archangel Gabriel coming to Mary who is in Nazareth (a city in Galilee) and declares to Mary that she will bear the son of God and call him "Jesus". The archangel greets Mary by saying, "Hail, full of grace" (Luke 1:28; "Hail, O favoured one" in other translations). The term "full of grace" ("gratia plena" in Latin) for Mary comes from the original Greek manuscript of the Gospel of Luke (who was a Greek himself). The word used by the archangel Gabriel (i.e. the word St. Luke uses for it) for the Blessed Virgin Mary was "kecharitomene" which translates to "having been graced" or "having been favoured". The archangel Gabriel is speaking in what's known as the "perfect passive participle", meaning Gabriel is talking about a quality that is and was always present, namely freedom from original sin.
How can this be explained in layman's terms? Mary's preservation from original sin is like this: if you fall into a pit and I pull you out from it, I've saved you. In the same way, if you're walking along and I prevent you from falling in the pit (either by warning you about it or physically stopping you before you get there) I have also saved you this way. In the latter instance, I have "preserved" you from the fall, ergo Mary has been "preserved" from original sin.
Mary's Immaculate Conception is not of her own doing (Catholics do not deify Mary) nor is it the doing of her parents, St. Joachim and St. Anne. Preservation from original sin is the gift given to Mary by God. Was it necessary that Mary be free from original sin in order to be the mother of the human person of God (Jesus Christ)? No, but it was fitting that she be preserved from it. Just as the Ark of the Covenant from the Old Testament (Exodus 25:10-22) was crafted according to detailed and specific instructions given by God, so too was Mary created with a purpose: with Christ, giver of the New Covenant, in mind. This is why we call Mary the "Ark of the New Covenant". To emphasise what this means, have a look at the following table and I will finish things with this to allow you to ponder these things within your own heart:
God bless you.