Detailed imagery of an English spring brings the speaker's homesick yearning to life—and lets the reader share that yearning.
Longing for home from "abroad," this poem's speaker seems almost transported by vivid memories. Take a look at this moment:
That the lowest boughs and the brushwood sheaf
Round the elm-tree bole are in tiny leaf,
Everything in this description is specific. The speaker seems to be getting right up close to the "lowest boughs" and the "elm-tree bole"—close enough to see the "tiny lea[ves]" that are only just beginning to peep out. The particularity of this image manages to conjure up all sorts of other details without even mentioning them: readers can practically see the lively green of those little leaves, and smell the fresh, damp earth.
A few lines later, the speaker gets even more particular:
Hark, where my blossomed pear-tree in the hedge
Leans to the field and scatters on the clover
Blossoms and dewdrops—at the bent spray's edge—
That's the wise thrush; [...]
The speaker doesn't just notice any old "pear-tree" here: it's "my blossomed pear-tree," a tree as familiar as an old friend. Describing this tree leaning over the grass to "scatter[...] blossoms and dewdrops" on the flowery field like gifts, the speaker again hints at even more sensory detail than the poem spells out—the sweetness of the "clover," the coolness of those "dewdrops."
Imagery even helps the speaker to spend the whole span of a morning in their imagined England:
And though the fields look rough with hoary dew,
All will be gay when noontide wakes anew
The buttercups, [...]
The image of fields "rough with hoary dew" conjures up an early morning when the grass is flattened down under the weight of the dew, its greenness muted. Here, the speaker's imagery works like a time-lapse camera: before the reader's eyes, that dew evaporates, the grass stands up straight, and the "buttercups" open under the warm "noontide" sun. The speaker, these lines suggest, has seen this wonderful phenomenon countless times, enough to be sure that "all will be gay" again.
The poem's imagery thus immerses readers, not just in a generic English countryside, but in the speaker's own touching memories of an English spring.