Hello!
The answer is: He wrote a letter describing the violence African Americans faced.
Why?
According to history, Martin Luther King Jr was arrested (and sent to jail) because he and other people were protesting against the violence and the bad treatment of black people in Birmingham, Alabama. A court ordered that Martin Luther King Jr. could not protest in Birmingham. There was a hard time for blacks living in Birmingham. King wrote a letter from jail describing the violence and bad treatment that blacks were facing in that time.