 <p>The columns in the output array from <code>predict_proba</code> are the probabilities of the different labels being predicted by your classifier. In your case, you've built a binary classifier, so the first column <code>modl.predict_proba(X_test)[:,0]</code> is the probability of the label being <code>0</code> and the second column <code>modl.predict_proba(X_test)[:,1]</code> is the probability of the label being <code>1</code>.</p>