And now I could attempt to summarize 2000+ years teaching regarding the Immaculate Conception of Mary. This would subsequently involve discussion of original sin, baptism, salvation, etc. Or I can link to Wikipedia.
The Immaculate Conception of Mary is a dogma of the Roman Catholic Church, according to which the Virgin Mary was conceived without any stain (in Latin, macula or labes, the second of these two synonymous words being the one used in the formal definition) of original sin. It is one of the four dogmata in Roman Catholic Mariology. It is completely distinct from the Virginity of Mary and the virgin birth of Jesus, though it is a popular mistake to confuse them. Mary is sometimes called the Immaculata (the Immaculate One), particularly in artistic contexts.Long story short: God designated Mary from her conception to be prepared to carry His son and today we celebrate that miracle and the fact God is awesome.
The proclaimed Roman Catholic dogma states "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 Saviour of the human race, was preserved free from all stain of original sin". Being always free from original sin, she was from the start filled with the sanctifying grace that would normally come with baptism after birth. Although widely-held since at least Late Antiquity, the doctrine was not formally proclaimed until Pope Pius IX did so in 1854 in Ineffabilis Deus.