title: What is the purpose of tf.global_variables_initializer?
body: <p>I would like to understand what <code>tf.global_variables_initializer</code> does in a bit more detail.
  A <a href="https://www.tensorflow.org/api_docs/python/tf/global_variables_initializer" rel="noreferrer">sparse description is given here</a>:</p>
  
  <blockquote>
    <p>Returns an Op that initializes global variables.</p>
  </blockquote>
  
  <p>But that doesn't really help me. I know that the op is necessary to initialize the graph, but what does that actually mean? Is this the step where the graph is complied? </p>