Metaphor is a literary figurative device/ language used by writers in making comparisons between two things, which may not be literally applicable for the issue addressed. But this comparison makes it more understandable or easier to comprehend.
In his "Letter from Birmingham Jail" of 1963, Martin Luther King used this metaphorical speech to address the issue of racial discrimination that his people are facing. He states that it will be "easy" for anyone who has never experienced the prejudice his people suffered, to say that the issue of racial "<em>segregation</em>" can be let alone and more important matters to be worked on. He compares this racial discrimination with that of a "<em>stinging dart</em>" which hits hard and stings everything it hits.