I love that my husband did this.
The Compass explains the meaning...
"The three letters have two different meanings. The first meaning is that they represent the initials of the three Wise Men who visited the holy child in Bethlehem. While they are not named in the Bible account (Mt. 2:1-12), over the years, tradition gave them the names of Caspar (spelled Kaspar in Poland), Melchior and Balthasar: C, M and B.
Another tradition gives the letters a Latin twist: Christus Mansionem Benedicat, or “Christ, bless this house.”"