Imagery in poetry refers to the pictures or images that are conjured up in the readers mind, by the words that a poet uses in his or her poem. For example, instead of saying "wealthy woman with a dog", a poet could say, " a diamond studded collar hung from the neck of her poodle" which would indicate wealth.
Through the use of words, the poet creates images that evoke meaning in the reader's mind.
The root meaning of 'imagery' is to 'imitate.' It literally means 'a reflection in a mirror.' Hence, an 'image' is 'a word picture' by means of which the poet conveys his feelings and emotions to his readers. When we come across an image in a poem it helps us to imagine and experience the same feelings and emotions which the poet experienced.
For example, Shelley in his "Ode to a Skylark" is captivated by the beauty of the bird but he is unable to make his readers comprehend its beauty in a few words, so he compares the skylark to a series of objects and hopes that the images he has created will help his readers to comprehend the beauty of the bird:
"What thou art we know not; What is most like thee? From rainbow clouds there flow not Drops so bright to see,
As from thy presence showers a rain of melody: -"
In this stanza he compares the beauty of the skylark and its soul stirring melody to the raindrops which form the rainbow.