IHS is a monogram or symbol of the name Jesus. It is a contraction of the Greek word for Jesus.

O my Jesus, thou art the Savior who hast given Thy blood and Thy life for me, I pray Thee to write Thy adorable name on my poor heart; so that having it always imprinted in my heart by love, I may also have it ever on my lips, by invoking it in all my necessities.

by St. Alphonsus Liguori