Everything you enter — moods, symptoms, feeds, all of it — lives only in this device's browser storage and is never uploaded. During the beta we collect anonymous usage analytics: which screens and buttons get used, so we can improve the app. Never the content of what you track — not a mood, a score, a measurement, or a word you type. Feedback additionally goes out only as an email you send yourself. Back your data up now and then from Settings.
Export saves a JSON backup; Import restores it (handy for moving to a new phone). Clearing the browser's site data erases the app's memory, so export once in a while.