title: Don't understand export javascript ES6
body: <p>I don't really understand what does the <code>export</code> keyword in ES6. Cause when I do something like the first exemple <a href="http://www.exploringjs.com/es6/ch_modules.html" rel="nofollow">here</a>, but without export anything in ES5 it's still works.</p>
  
  <p>//------ lib.js ------</p>
  
  <pre><code>var sqrt = Math.sqrt;
  function square(x) {
      return x * x;
  }
  function diag(x, y) {
      return sqrt(square(x) + square(y));
  }
  </code></pre>
  
  <p>//------ main.js ------</p>
  
  <pre><code>console.log(square(11)); // 121
  console.log(diag(4, 3)); // 5
  </code></pre>
  
  <p>So of course I know that I miss a big part, that's why I'm looking for some explanation or link that explain what I need to learn cause I don't find anything :)</p>