The SentimentEvaluator function uses test data to evaluate the precision and recall of the predictions output by the function SentimentExtractor. The precision and recall are affected by the model that SentimentExtractor uses; therefore, if you change the model, you must rerun SentimentEvaluator on the new predictions.
For basic information on precision and recall calculations see http://en.wikipedia.org/wiki/Precision_and_recall.
SentimentEvaluator uses files that are preinstalled on the ML Engine. For details, see Preinstalled Files That Functions Use.