Brief Summary
The Set-Up
Dr. King was arrested and sent to jail for protesting segregation in Birmingham, Alabama. After reading an open letter from eight white clergymen in the local newspaper criticizing him and his fellow activists, MLK decided he might as well write back to let them know what was on his mind. Enter: letter to the editor.
The Text
Turns out he had a lot on his mind, and that he hadn't just shown up to the protest to get out of preaching to his congregation for a few days. He had a philosophy and a plan and everything.
Dr. King used the opportunity to bring everybody up to speed about the protests in Birmingham, what they were about (horrible systemic racism), why the protestors were civilly disobeying (racist) laws and ordinances, why the protestors had truth and justice (and Jesus/America) on their side, and how Dr. King was disappointed with clergymen in the South and so-called white moderates who supposedly believed in his cause but didn't like the "tension" and unrest caused by the protests.
TL; DR
Get off the couch, get out in the streets, and protest injustice with civil disobedience and love.