The following articles will answer your question:
catholic.com/library/Mary_Full_of_Grace.asp
catholic.com/library/Immaculate_Conception_and_Assum.asp
From another thread:
The Koine Greek used at the time is different from modern Greek. It is a much more precise language than English. The words used of Jesus in John 1 are Pleres Charis. Pleres means “full” or “lacking nothing” (and is applied also to “truth”). Charis simply means “Grace”.
The word used of Mary in Luke 1 is Kecharitomene. It is a complex participle. The root of this word is Charis, meaning Grace. The prefix ‘Ke’ means that the grace was already perfectly present before the angel appeared. The suffix ‘mene’ means that Mary was the recipient of this grace. In other words Kecharitomene shows that Mary was a recipient of a fullness of Grace. The grace was given to her. The perfect tense indicates that this filling with grace is a continuous event that was ongoing before the angel appeared.
The translations which state “highly favoured” instead of “full of grace”, are largely motivated by a doctrinal desire to downgrade Mary. The first version to use this translation was the King James Version, where the translators noted that “highly favoured” was used instead of “full of Grace” in order to show that Mary was not a source of grace. (Tyndale’s version on which the KJV was largely based, used full of Grace).
Most protestant bibles followed the KJV, (along with some modern Catholic ones)
Now, in modern Greek, which Protestants used to interpret these verses, Charis can be translated simply as “favour”. So “Highly-favoured” could be a conceivable translation - but this would only be acceptable if the word “favour” were used as a translation for “Charis” everywhere else in the New Testament. But THIS DOES NOT HAPPEN. Even those bibles which translate “Charis” as Favour" when referring to Mary, translate it as “Grace” everywhere else. This is highly misleading because in the New Testament the word “Grace” has a particular meaning distinct from “Favour”.
In the New Testament “Grace” is a gift of God that saves from sin and its effects. So translating the word any differently is wrong. The correct translation is rightfully “Full of Grace”.