The stemming process transforms words into their root form, so inflected and derived words are grouped together. For example the words fishing, fished or fisher are transformed to the root word fish.

This is enabled by default if a particular language is selected, usually it will help the classifier generalize and improve the classification, but it depends on your data and what are you trying to do with it. 

You can turn stemming on and off to see what impact it has on your classifier.