Imagery is a literary device which appeals to one (or all) of the five senses (sight, sound, taste, touch, smell). Authors use imagery to appeal to the senses of a reader so that they can experience a deeper connection with the text.
In Emily Dickinson's poem, XXI from Part One of "Life", "He ate and drank the precious words" imagery is used to appeal to a reader's sight.
The poem depicts a man sumbolically drunk off of words he has read in a book. The book has not only "intoxicated" him, it has allowed him to forget the problems in his life as well.
The imagery of the poem details an image of a man, drunk and dancing. This is a visual image that the reader can create in their own mind.