The Immaculate Conception is a Catholic dogma that states that Mary, whose conception was brought about the normal way, was conceived without original sin or its stain. That’s what “immaculate” means: without stain.
It’s important to understand what the doctrine of the Immaculate Conception is and what it is not. Some people think the term refers to Christ’s conception in Mary’s womb without the intervention of a human father; but that is the Virgin Birth. Others think the Immaculate Conception means Mary was conceived “by the power of the Holy Spirit,” in the way Jesus was, but that, too, is incorrect.
The essence of original sin consists in the deprivation of sanctifying grace, and its stain is a corrupt nature. Mary was preserved from these defects by God’s grace; from the first instant of her existence, she was in the state of sanctifying grace and was free from the corrupt nature original sin brings.