The monogram (initials) IHS refers to Jesus and is made up of the first three letters of the name of Jesus when written in the Greek alphabet.  The three letters were incorrectly transcribed/written in the Latin/English alphabet as I H S.  The monogram became very popular in the Middle Ages and many explanations of IHS were offered, the most common being Iesus Hominum Salvator (latin for “Jesus, Saviour of Mankind”).  Likewise, today IHS is sometimes understood to represent the phrase “In His Service”.  The monogram is frequently seen in liturgical art and architecture, especially vestments.