title: What does it mean to "join" a thread?
body: <p>For my class I'm supposed to find out what's wrong with a piece of code, and the part I'm having trouble deciphering is</p>
  
  <pre><code>// joining a thread blocks until that thread finishes
  a.join();
  b.join();
  </code></pre>
  
  <p>Is joining a thread the same as locking a thread?  Because I think the point of this assignment is you're not supposed to leave threads unlocked.</p>