The curfew tolls the knell of parting day, The lowing herd wind slowly o'er the lea, The plowman homeward plods his weary way, And leaves the world to darkness and to me.
So, right off the bat we have some vocab to sort out in this poem. The "curfew" is a bell that rings at the end of the day, but a "knell" is a bell that rings when someone dies. So it's like the "parting day" is actually dying. Sounds like a metaphor!
The mooing herd of cows makes its winding way over the meadow ("lea" = "meadows")
And the tired farmer clomps on home.
Now that the cows and the farmer are out of the picture, the speaker gets everything in the world to himself (he has to share it with the growing darkness, but that's not so bad).
Notice that the speaker refers to himself in the first person right away in that first stanza: the parting farmer and cows leave "the world […] to me."
This would be a good time to note that the poet often removes vowels and replaces them with an apostrophe, like "o'er" instead of "over" in the second line.
If you ever notice an odd-looking word with an apostrophe in it, try replacing the apostrophe with a letter to make a familiar word. Gray makes these contractions to make the number of syllables fit the iambic pentameter. While we're talking about form, we'll also point out the rhyme scheme here—it's ABAB. For more on the poem's meter and rhyme scheme, check out the "Form and Meter" section.