There are many different kinds of poetic devices. In general, poetic devices imbue literal depictions with figurative meanings. They can also add an extra flair to the style of writing in order to help make the author's point.
Although To Kill a Mockingbird is written in prose, Harper Lee uses numerous poetic devices throughout it. About halfway through chapter 18, a metaphor is used to describe how Atticus moves about the courtroom while questioning Mayella Ewell.
Atticus was making his slow pilgrimage to the windows, as he had been doing: he would ask as question, then look out, waiting for an answer.
Remember that a metaphor is a comparison of two unrelated things. A word or phrase is applied in a way to mean something other than its literal meaning. A pilgrimage is a long journey made as an expression of religious devotion. Of course, Atticus is not on a literal pilgrimage. On the face of it, he is merely walking around the courtroom. However, the use of this metaphor as a poetic device underscores the deliberate nature of Atticus's actions. It adds a sense of vividness to the scene that would not be achieved if Atticus were simply described as walking around.