title: The interpretation of cross validation scores
body: <p>I am trying to understand this part of code that I found on the Internet:</p>
  
  <pre><code>kfold = KFold(n_splits=7, random_state=seed)
  results = cross_val_score(estimator, x, y, cv=kfold)
  print("Results: %.2f (%.2f) MSE" % (results.mean(), results.std()))
  </code></pre>
  
  <p>What the does <code>cross_val_score</code>?
  I know that it calculates scores. I want to understand the meaning of these scores, and how they are evaluated.</p>