This is a monogram of the name of Jesus Christ. From the 3rd century the names of Christ were sometimes shortened, particularly in Christian inscriptions (IH and XP, for Jesus and Christus). In the 4th century the "sigla" (chi-rho) occurs not only as an abbreviation but also as a symbol.
From the beginning, however, in Christian inscriptions the names of Jesus Christ, were shortened by contraction, thus IC and XC or IHS and XPS for Iesous Christos. These Greek monograms continued to be used in Latin during the Middle Ages.
IHS is the Christogram for the Greek spelling of Jesus (ΙΗΣ- iota-eta-sigma; short for ΙΗΣΟΥΣ)