Metaphor
Basically, a metaphor is figure of speech that likens two things that happen to be quite different for the sake of presenting a clearer image to readers. What the author does is anticipate what readers will be familiar with and compare that something to what readers may not be too familiar with. This also helps make content easier for readers to understand. For instance, in this excerpt, because readers were not present to see just how much the pins were over the man, the author likens the pins to what most readers are probably familiar with—a rash. Thus, the author is implying that if you know just how badly a rash can cover a person, then you can begin to comprehend just how rampant the pins were all over this person.