Regina Caeli means “Queen of Heaven.” As mother of the King, Mary holds the title of Queen Mother. At Vatican II, Mary was called Queen of the Universe. Not too shabby for a poor girl from backwoods Judea. The title Queen of Heaven is often associated with her assumption into heaven; it is said that she assumed and then was crowned. She is often depicted as the woman in Revelation with the moon under her feet and a crown of stars on her head.
In the Old Testament, the Queen Mother had persuasive power in court. Solomon tells Bathsheba that he will not refuse her request (1 Kings 2:20). Similarly, Jesus performs his first miracle at Mary’s request (John 2:3-6). This courtly power of the Queen Mother is one of the reasons Mary is such a popular intercessor; people ask for her prayers because in heaven, her requests will not be refused.
The Liturgy of the Hours has antiphons to Mary the Queen, including the anthem Regina Caeli. My favorite is the Salve Regina (Hail Queen).