A metaphor is a figure of speech in which it is not the literal meaning, for example, she has a "heart of gold." She doesn't literally have a "heart of Gold," but an author will say this to give us a beautiful way to describe a character trait of a character.
On lines 6-7, the metaphor of the wall is being compared to a scar on skin. What could a possible reason be of why the author, Rios, uses such a metaphor?