Some village-Hampden, that with dauntless breast The little tyrant of his fields withstood; Some mute inglorious Milton here may rest, Some Cromwell guiltless of his country's blood.
The speaker muses that there might be dead people buried here that could have been famous revolutionaries or poets, but they died unknown and undiscovered.
Maybe there was some village-version of John Hampden who stood up to tyranny on the village green!
(Historical side note: the real John Hampden was a Puritan politician who opposed the policies of King Charles I. He refused to pay a tax he thought was unfair. So Gray calls him "dauntless," or "fearless," for standing up to the "little tyrant," or the king.)
Or maybe there was someone as brilliant as John Milton (you know, the guy who wrote Paradise Lost), but he died mute, without being able to express his brilliance.
Or maybe there was someone who would have wreaked as much havoc as Cromwell, but who didn't have a chance.
Another historical note! Oliver Cromwell was the leader of the anti-royalists during the English Civil War, helped bring about the execution of King Charles I, and became head of the short-lived English Commonwealth in 1649-1660. He wasn't a popular guy in the history books at the time Gray was writing.
Another fun fact! Both Hampden and Milton were from the same area of England where Gray was writing his "Elegy." So maybe Gray liked to imagine that the same area could have produced other guys who were just as brilliant, but who remained unknown.