When you record audio and make notes using RecordBox for iPhone and iPad, all of your data is stored only on that device by default. When a Page or Track is stored only on one device, it is marked as 'Not Synced', as shown below:
When you press 'Sync Track' or 'Sync Page', RecordBox will create a copy of your Pages, Track, and Notes on the RecordBox servers. You will then be able to access your data from multiple devices, amongst other things. Have a look at Why should I sync my tracks? for more information.