A Christogram is a monogram or combination of letters that forms an abbreviation for the name of Jesus. They are traditionally used as a religious symbol within the Christian Church.
After images of the cross, in the Latin-speaking Christianity of medieval Western Europe, the most common Christogram was ‘IHS’. These three letters represent the three consonants of ‘Jesus,’ when written in Greek: Jesous = ΙΗΣΟΥΣ; and this name starts iota-eta-sigma, or ΙΗΣ. (In Latin, the letters I and J were not distinguished so ‘JHS’ and is the same as ‘HIS’.) So we are being invited to bring Jesus to mind each time we see the letters IHS.