Imagery is when an author uses words to paint a mental "image" in the mind of the reader. It is defined fully 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 Nathaniel Hawthorne's The Scarlet Letter, an example of imagery can be found in Chapter Ten, "The Leech and His Patient," as Hester Prynne and her daughter Pearl move through the grave yard. Pearl comes to the attention of Roger Chillingworth and Mr. Dimmesdale:
So she drew her mother away, skipping, dancing, and frisking fantastically, among hillocks of the dead people, like a creature that had nothing in common with a bygone and buried generation, nor owned herself akin to it.
This is a much more "colorful" way to describe a young child running through the cemetery, unconcernedly, with no thought to the graves she is passing over...even unaware that she is a mortal being as are those buried beneath her feet.