If the device has an online connection, data is stored locally and sent via the internet right away.
If the device is currently offline, data is stored locally and can be synced later when an internet connection is present.
For android users you have to press the "Sync" button located in the main menu.
For iOS users the app will automatically keep the device synced. If a manual sync is required you can go to the "More" tab and select the "Account and Sync" option. Press the "Sync now" button.