The poem begins with the speaker metaphorically describing a (presumably pink) flower that they come across in the woods as a "cheek." This flower is dead, or just about: its once "rosy face / Has lost a blush." The flower hasn't yet decayed, but it no longer glows with the same bright color that it did only recently; blood no longer rushes to this metaphorical "cheek."
The speaker talks about the flower as if it were a human being, and this personification makes the poem's imagery more striking and urgent. By using the words "cheek" and "rosy face," as opposed to, say, "petal" and "flower," the poem implies that the speaker isn't just talking about a literal plant here. This little flower becomes a symbol of vulnerable beauty in general, and perhaps of the fragility of life itself.
Note, too, how instead of saying "who is this" or "what is this," the speaker asks "Whose cheek" this flower belongs to and "what rosy face" has lost its color. Such language subtly reflects how the flower has been severed from its life source and now lies isolated and vulnerable in the woods. The enjambment between "face" and "has" mirrors the flower's separation from its roots/stem/branch.
The poem's language and form further evoke the flower's vulnerability. These lines are very short—two lines of iambic dimeter (da-DUM da-DUM) followed by one of trimeter (da-DUM da-DUM da-DUM):
Whose cheek | is this?
What ros- | y face
Has lost | a blush | today?
These brief, largely monosyllabic lines also feature lots of delicate sibilance ("rosy face," "lost a blush"). Overall, the poem feels hushed, gentle, and reverent.