When you turn editor tracking on in a ArcGIS Online feature service, your features will get four new hidden fields in the background which holds the information. They track the creator name, creation date, editor name, and edit date. You will be able to see these fields in the table for your features within a web map. If you want editor tracking from ArcGIS for Desktop I believe you have to publish the feature service with editor tracking through a server, not on ArcGIS Online.
This is something I am currently experimenting with myself as I don't want the four automatically created tracking fields. I need a way to utilize exiting fields within my features to track usernames and dates and all my research points to needing to publish the feature service via a server.