title: What do classes tf.train.Coordinator and class tf.train.QueueRunner do in tensorflow?
body: <p>I understand that both classes deal with threads. According to the documentation, tf.train.Coordinator coordinates the termination of a set of threads and tf.train.QueueRunner holds a list of enqueue operations for a queue, each to be run in a thread. </p>
  
  <p>However, what is their role in simple words? When are they necessary during the training?</p>