Beneath those rugged elms, that yew-tree's shade, Where heaves the turf in many a mould'ring heap, Each in his narrow cell for ever laid, The rude forefathers of the hamlet sleep.
This stanza is all one long sentence, and the sentence structure is a bit wacky, so let's try to sort it out.
The subject and the verb of the sentence are way down there in the last line of the stanza: "The rude forefathers of the hamlet sleep."
Hold up—the speaker isn't saying that the ancestors of the town (a "hamlet" is a tiny town, not an omelet with ham in it!) are impolite. "Rude" is used to describe someone who was from the country. Someone who wasn't sophisticated, and who was maybe a bit of a bumpkin. So the forefathers being described here are probably just simple country folks, not discourteous, impolite jerks.
So what are these country forefathers of the hamlet doing? They're sleeping. Sounds peaceful, right?
Except, look at the third line of the stanza—they're not sleeping at home in their beds. They're sleeping in narrow cells, and they're laid in there forever.
Sounds like they're sleeping in only a metaphorical sense. These guys are dead and lying in their graves in the churchyard!
The first two lines of the poem set the scene. These graves are under elm and yew trees, and there are piles of turf on each one.
So we're not just hanging out outside of a church as the sun goes down. We're actually hanging out in the graveyard. Spooky!