It's a keystone, a Masonic emblem (probably for a Master Mason - or a Mark Master). The letters stand for "Hiram The Widow's Son Sent to King Solomon," which has to do with the founding myth of masonry: that the first mason, Hiram Abiff, came from Tyre as an architect to help Solomon build the first Temple, and is killed by three craftsmen who are after his secrets... but is either resurrected or exhumed and reburied with honor. The name "Hiram," according to the Masons, comes from a root that means "lifted up" or "exalted."

There a whole set of allegorical readings of that story which probably isn't what you're looking for. It's based on (or mentioned in) 1 Kings 7, where Hiram/Huram is identified as a widow's son with great skill and understanding who does all kinds of elaborate work on the Temple, which is the house that God kinda sorta literally, physically moves into at this point in the Hebrew Bible.

There's more about the acronym - with a couple of different variations of what it really stands for - here at this Masonic education site, although if you want heavier reading, there's always The Morals and Dogma of the Ancient and Accepted Scottish Rite of Freemasonry.

Oh, and elsewhere on the headstone, I think you'd likely see either two or five columns - the two columns are named Jachim and Boaz and have, again, a wealth of symbolism behind them. The keystone unites them.