The cloud library automatically keeps your Public and Private items synced with the cloud. Although your items are also stored locally, any change will automatically by sent to their cloud version. This secures your items from being lost if you loose your phone and avoids the hassle of manually re-uploading smaller changes.
It also allows you to work on multiple devices, as new or changed items will be automatically downloaded.
Whenever the library is synchronizing your items, it will show you a sync bar with information about the current progress. Synchronization happens in the background, so you don't have to keep the library open until it is finished.
You can trigger a re-sync using the Force Library Resync option in the overflow menu of the app bar.
If a full resync is manually or automatically triggered and you are currently not on Wi-Fi, the sync bar will ask you if you would like to sync anyway. To do so, tap the sync bar.
If you would like to bypass this behavior and allow re-sync at any time. Go to Settings .. Cloud library .. and disable the option Full synchronization requires Wi-Fi.