Progressive revelation refers to the idea that later writings build on those that come from earlier times, adding new meanings or expanding on previous messages. The record of God giving Moses the Ten Commandments is first presented in Exodus. The book of Leviticus greatly expands the rules which the Jewish people lived by, as derived from those Ten Commandments.
Many passages in the Old Testament are seen as starting points in a progressive revelation of the coming of the Messiah. Micah 5:2 reads
But you, O Bethlehem Ephrathah, who are little to be among the clans of Judah, from you shall come forth for me one who is to be ruler in Israel, whose origin is from of old, from ancient days.
Isaiah 7:14 reads
Therefore the Lord himself will give you a sign. Behold, a young woman shall conceive and bear a son and shall call his name Immanuel.
Christians read these and other passages as being progressive revelations, or prophecies, pointing to and confirming that Jesus is the one sent by God.