What is the meaning of IHS often seen on tombstones?

Take your pick....

In Hoc Signo
Iesus Hominum Salvator
Iota Eta Sigma

https://en.wikipedia.org/wiki/Christogram

I.H.S. ( Does not mean In his service) means one of three sayings "In Hac Salus" = In This Safety "Iesus Hominum Salvator: Jesus -Man- Savior the saver of men
"In Hoc Signo" - In This Sign" Ye Will Conquer (usually with a sword somewhere & shield) It is usually shown with a cross and circle together - Christian Cross or St. Andrews Cross and the circle of eternity)

http://newtheologicalmovement.blogspot.in/2012/01/what-does-ihs-stand-for-meaning-of-holy.html

More commonly is “I.H.S.” or “I.H.C.” found. The “I” stands for Ihsus or Ihcuc, the name of Jesus in Greek with the S and C being variant forms in the Greek alphabet. When these are found on tombstones it stresses the identity of the individual with Jesus Christ. “I.H.S.” or “I.H.C.” are often confused with the Latin phrase, In hoc signo. On the eve of battle, Constantine, had a vision of a banner on which the phrase, “In hoc signo vinces” was written. Its meaning being “In this sign you will conquer”. Constantine won the battle and converted to Christianity therefore causes some confusion when viewed in a graveyard marker.

[Hacker, 2001]

https://www.christianhistoryinstitute.org/incontext/article/constantines-cross/

First three letters of "Jesus" in Greek and the most common Christogram in medieval Western Europe; featured in the seal of the Society of Jesus (Jesuits).

https://en.wikipedia.org/wiki/IHS