What does king mean when he says shallow understanding from people of good will is more frustrating than absolute misunderstanding from people of ill will?

King means that those who have good intentions -- but only a superficial understanding of a cause or movement -- cause more damage than those who oppose a cause.

King explains that these people, people who have good intentions but poor understanding, often hurt the movement more than people who stand in direct opposition to his beliefs. He uses the example of moderates who agree with King's goals but not his methods. King argues that such people, who advise King to "wait for a 'more convenient season'" are a greater stumbling block to civil rights than those who totally oppose the movement.

Because these people have good intentions, their lack of understanding is more painful and frustrating than those of ill will.