cross_val_score meaning
The source, around line 274 is where the default scoring for cross_validation_score gets set, if you pass in None for the scorer argument.  For classifiers, the usual default score is accuracy.  For regression, it's rmse, IIRC.  So, since you're applying a decision tree classifier, cross_val_score splits the data into 5 equalish sized pieces, trains on each combination of 4 and gives back the accuracy of the estimator on the 5th.  The mean and std of these accuracies presumably tells one something about the performance of the family of decision tree classifiers on your dataset, but I would take it with a grain of salt.