The Extract key phrases transform extracts key phrases from a String attribute and creates a list of phrases in a new multi-assign attribute. The transform calculates key phrases using the TF/IDF algorithm, which takes the total number of times each term appears within the String and offsets that value by the number of times it appears within a larger body of work.
If you are done making changes to the project data set, you can commit the changes. See Running the transformation script against a project data set.