Images are words or phrases meant to appeal to the senses. Good images intensify the reader's interaction with the poem and help the poet convey his/her meaning or feeling.
To create unity poets often create a pattern of multiple images based on a central idea. In "The Road Not Taken" Robert Frost uses images from the woods that speaker is looking at. Each of the four stanzas in the poem contains some sort of image from these woods:
In stanza one he uses "two roads . . . in a yellow wood," and "undergrowth."
In stanza two he uses "grassy" and "worn."
In stanza three he uses "leaves no step had trodden black."
In stanza four he returns to the original image from stanza one with "two roads diverged in a wood."
Notice that the images in the pattern Frost creates are all related to the woods that he mentioned in the first stanza. In the first three stanzas each image adds a layer of thematic value to the poem's message. By repeating the "two roads" image in the final stanza, Frost brings the message full circle, giving the poem a sense of unity and completeness, as the poet has reached an uneasy acceptance the idea that he must make a choice and live with the consequences.