Theory:

Imagery is a technique used in literature. It is one of the several poetic devices used by the poets. Poetic devices are used to create a structure and meaning to the poetry.
So what is the role of Imagery?
Imagery means the usage of verbal images to describe a concept. The goal of using “imagery” is to create images in the minds of the readers. Writers use descriptive words to do that. The poem is said to have imagery if it describes the look, sound, smell, taste, and/or feel of something and these five senses are targeted while creating imageries. It helps the readers understand the literary work better.
So how is this relevant to the poem "On the Grasshopper and Cricket"?
The poem "On the Grasshopper and Cricket" has several imageries in it. While reading the poem, we can easily imagine the scenes described in the poem. The words such as sun, trees, birds, and voice are various images created in the minds of the readers.
Example:
The first stanza creates an image of scenery with trees, birds, sun in the readers' mind.
 
"voice will run from hedge to hedge".
all the birds are faint with the hot sun,
And hide in the cooling trees . . .
"rests at ease beneath some pleasant weed."
 
stanza.jpg
 
The second stanza can create images such as a winter, shrill sound of a cricket, and cold silence in the reader's mind.
 
On a lone winter evening, when the frost
Has wrought a silence, from the stone there shrills
The Cricket's song . . .
  
winter.jpeg