Imagery is the use of words to convey sensory experience. It can be visual (describing something one can see), auditory (describing something one can hear), tactile (describing something one can touch), olfactory (describing something one can smell), or gustatory (describing something one can taste). The imagery used in this poem is predominantly visual imagery. When the speaker of the poem describes
. . . the ground almost covered smooth in snow,
But a few weeds and stubble showing last
he employs visual imagery. We can imagine the field almost completely made smooth-looking by the blanket of white snow, while, here and there, the tops of a few plants are still visible poking through that smooth blanket. Later, the speaker describes
A blanker whiteness of benighted snow
With no expression, nothing to express.
This visual image describes the way that same snow looks at night, when any such minor imperfections in the whiteness become invisible in the dark. The smooth whiteness, then, seems even "blanker" than it does during the daytime.