The Use Stemming setting sets if words should be stemmed. 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 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 always edit this setting, retrain and check out the stats to decide what suits your case.