title: Action mustUnderstand header someone please explain it
body: <p>I've been trying to find a simple explanation on the web on this header. </p>
  
  <p>What is its use? 
  When do we need it? 
  What does it do?
  What does it mean?
  and so on...</p>
  
  <p>I know this is pretty simple but I've been looking for a well explained answer regarding this header online for more than an hour. </p>