Their name, their years, spelt by th' unletter'd muse, The place of fame and elegy supply: And many a holy text around she strews, That teach the rustic moralist to die.
The "frail" monuments (78) are engraved only with the dead people's name and the years of their birth and death, and even this simple inscription was clearly made by someone who was largely illiterate, or "unlettered."
The speaker uses irony when he says that inscription was made by a "muse." Since the muses were goddesses of poetry, how could they be unlettered or illiterate?
These simple inscriptions take the place of fame and fancy elegies (poems written in memory of dead people).
"She" is the muse referred to in the previous line.
The "unlettered muse" also adds ("strews") the occasional Bible verse ("holy text") that inspires country folks to think about death so that they'll be prepared when their time comes.