title: What does "Challenge" term stand for?
body: <p><code>ControllerBase</code> class has <code>Challenge</code> method, that returns an object of the <code>ChallengeResult</code> class.
  <code>CookieAuthenticationOptions</code> class has <code>AutomaticChallenge</code> property.</p>
  
  <p>I believe <code>ChallengeResult</code> has something to do with external logins. But how does it actually work? Where does the term "Challenge" come from? What does lay inside this.</p>