The poem takes place around the time of sunset in a country churchyard—also known as a cemetery. Kinda spooky, right? And the darkness of the setting is appropriate for the subject matter, too. The speaker is talking about the unknown. He's contemplating mortality and what happens to people after they die. Of course, no one really knows what will happen after death, so the darkness might symbolize the mystery of what happens after we die.
Line 1: The speaker uses personification in the very first line when he says that the church bell "tolls the knell" of the day. When a person dies, you ring a church bell to commemorate their death, and that's called a "death knell," so the poet is implying that the bell that rings at sundown is commemorating the death of the day, as though the day were a real person.
Lines 5-6: The speaker uses alliteration, or the repetition of consonant sounds, when he describes the "solemn stillness" of the scene at sunset. The repeated S sound (also known as sibilance) is like a sort of "shushing"—maybe the speaker wants to emphasize the quiet, calm, stillness of the atmosphere.
Lines 13-16: The speaker uses a metaphor when he says that the dead villagers are only "sleeping" in the shade of the tree. In fact, this is a euphemism, or a polite way of describing something to soften its harsh reality (like saying that you're "excusing yourself for a moment" at a fancy dinner, rather than saying "I have to go pee now"). Why would Gray use a euphemism here? Could be that part of him is afraid of death and his own mortality, so he'd rather think of these villagers as merely "sleeping" or resting comfortably, rather than rotting away underground?
Lines 53-54: The speaker uses a metaphor when he describes people whose good qualities go unrecognized as "gems" that are hidden in dark caves under the ocean.