Maps all words to their canonical forms for easier analysis. For example, the canonical form of "running" and "ran" is "run". This uses the CoreAnnotations.LemmaAnnotation annotation from Stanford CoreNLP.
For more information, please refer to http://nlp.stanford.edu/software/corenlp.shtml or Manning, Christopher D., Surdeanu, Mihai, Bauer, John, Finkel, Jenny, Bethard, Steven J., and McClosky, David. 2014. The Stanford CoreNLP Natural Language Processing Toolkit. In Proceedings of 52nd Annual Meeting of the Association for Computational Linguistics: System Demonstrations, pp. 55-60.