title: What does FragmentManager and FragmentTransaction exactly do?
body: <p>I have simple code below</p>
  
  <pre><code>FragmentManager fragmentManager = getFragmentManager();
  FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
  fragmentTransaction.replace(R.id.fragment_container, mFeedFragment);
  fragmentTransaction.addToBackStack(null);
  fragmentTransaction.commit();
  </code></pre>
  
  <p>What do these lines of code do?</p>