The word "magi" means wise men. The term is used to refer to the three wise men who came from the east to worship the newborn Jesus in some of the Gospels.
The word is used in the title sort of as irony and also to refer to the idea that Jim and Della are actually wise.
The word is ironic because the wise men had really splendid gifts to give Jesus -- gold, frankincense and myrrh. They were clearly rich. But Jim and Della don't have hardly anything to give one another.
The word also refers to Jim and Della being wise for being so in love and appreciating each other so much.