A St. Christopher necklace is a chain with a medallion bearing the image of Saint Christopher. He is considered the Patron Saint of Travelers. The necklace is usually worn by Catholics or those seeking protection for trips.
The St. Christopher necklace medal usually bears the image of a tall, bearded man carrying a small child on his back. He may or may not be crossing a river in the image. Around the image is the phrase, "Saint Christopher, Our Protector," in Latin.
According to legend, St. Christopher was a very tall and strong man who served the King of Canaan. Christopher believed that his king was the greatest of all, but one day he learned that the king feared the devil. Wanting to serve the greatest master, Christopher sought out the devil to serve and found a band of raiders whose leader declared himself the devil. He spent some time serving the leader of these raiders, until Christopher saw the leader avoid a cross on the side of the road and realized the man was afraid of Christ. So, Christopher decided to seek out and serve Christ.
The Saint did this by ferrying people across a dangerous section of river, where many would have drowned. One day, Christopher carried a small child across the river. On the other side of the river, the child revealed himself to be Christ, showing that St. Christopher was literally serving Christ.