Imagery can be defined as
the forming of mental images, figures, or likenesses of things. It is also the use of language to represent actions, persons, objects, and ideas descriptively.
In order for imagery to be used effectively in a written piece of literature, the reader needs to be familiar with the likenesses or langauge being used to create the image. A reader who has no familiarity with the terms or concepts being used in creating the image will not understand correctly the meaning being conveyed by the author.
Consider the poem "The Fog," by Carl Sandburg: "The fog comes
on little cat feet. It sits looking over harbor and city on silent haunches and then moves on." The imagery of the fog quietly sneaking into the "harbor and city" and silently disappearing is unmistakeable.
Someone reading this poem who has seen cats move about will be able to visualize the way in which the fog comes into an area and then moves away from it easily. A reader who has never watched a cat will gain little understanding of how fog moves from reading this poem.