There is a good explanation of this poem right here on eNotes, at the link below.
Essentially, the poem can be summed up by the following line (from the poem):
The paths of glory lead but to the grave.
Note that the poem is "written in a COUNTRY churchyard." This poem, therefore, is about the deaths of common people, farmers, workers, mothers and fathers:
Oft did the harvest to their sickle yield,
Their furrow oft the stubborn glebe has broke;
How jocund did they drive their team afield!
How bow'd the woods beneath their sturdy stroke!
An "elegy" is a poetic lament for the dead, and this poem celebrates the honor of lives that were lived simply, but happily. Just because the people buried in this country churchyard were common people, does not mean that their lives were not important in God's eyes.
As the epitaph on the gravestone says:
No farther seek his merits to disclose,
Or draw his frailties from their dread abode
(There they alike in trembling hope repose),
The bosom of his Father and his God.
Death is a type of rest in which nothing one has accomplished on earth matters. We leave our hopes, our dreams, our "frailties" and our "merits" at the grave as we enter into the "hope and respose" of the Father, God.
That Thomas Gray's "Elegy Written in a Country Churchyard" is a powerful and poignant poem is evinced in its immediate success, as well as in the many imitations of this work. In fact, Samuel Johnson declared Thomas Gray the man who wrote the English poem most loved by "the common reader."
Gray felt that "the language of the age is never the language of poetry." Yet, although he uses archaic diction and distorted syntax at times, Gray's elegy balances Latinate phrases with current English expressions. Moreover, thematically it touches a common humanity that all readers can share. Johnson, who did not care for Gray's poetry, recognized this elegy as one that would last forever:
The churchyard abounds with images which find a mirror in every mind, and with sentiments to which every bosom returns an echo.
In the neo-classical form of an elegiac poem, Gray expresses his individual estimate of the world using eloquent classical diction. The verses carry a lofty tone and are composed of heroic quatrains (four lines of iambic pentameter with the rhyme scheme abab). The neo-classical use of personification abounds in this formal work, as well, as in the following stanza, which also exemplifies Gray's lofty tone:
Let not Ambition mock their useful toil,
Their homely joys, and destiny obscure;
Nor Grandeur hear with a disdainful smile
The short and simple annals of the poor.
What lends the poem its beauty and poignancy is the moving expression of thought and emotion that is purely Romantic, as it touches upon Nature and sympathy for the unknown in the graveyard. The idea that
Chill Penury repressed their noble rage,
And froze the genial current of the soul
is a poignant one, indeed, as Gray raises the implicit question of social class at a time when ideas of equality were exceptional. Also, the sentiment of remembrance for the "unhonored dead" who have waged battles of their own but their "lot forbade" their renown is brightened by the Romantic notion that Nature and the Eternal provide hope after death, as the soul reposes in "The bosom of His Father and his God."
This line about the bosom of God is the last of the elegy's epitaph. Traditionally, this elegy has begun solemnly with the poet's lament, but ends with an insight that enables the poet to cope with the loss he senses.