By default, the App Engine Datastore writes two index records for every entity property that isn't a com.google.appengine.api.datastore.Blob or a com.google.appengine.api.datastore.Text. These index records allow you to execute a variety of queries involving the property without creating a composite index.
These indexes take time to write and they take up space on disk. If you have a property that you're absolutely positively sure you'll never want to filter or sort by, you can opt-out of the default indexing that is going on.
Continue to Automatic Values.