The Magi, also known as the "Three Wise Men," are figures from the Bible who, following a star in the sky, traveled from the East to Bethlehem to worship the newborn baby Jesus, offering him gifts of gold, frankincense, and myrrh as symbols of his kingship, divinity, and mortality; their story is recounted in the Gospel of Matthew. 
The names and lands of origin of the Magi were eventually assigned to them, but they were not consistently assigned. In Western church tradition, Balthasar is often depicted as a king of Arabia or Ethiopia, Melchior as a king of Persia, and Gaspar as a king of India. The gifts the Magi brought are said to have symbolic meanings: gold represented Jesus' status as the "King of the Jews", frankincense represented his divinity, and myrrh represented his mortality. 
The church has traditionally suggested that the 12 days represent the time between the birth of Jesus and the arrival of the Magi. The days start on Christmas day (or in some traditions, the day after Christmas) and culminate with the feast of Epiphany, which is observed on January 6th.