Your mobile device can synchronize files from other devices in two ways:
Selective Sync is OFF
When you disable Selective Sync on your mobile device, ALL the files will be synchronized to this device automatically. If any of the files are changed on the device or new files are added, they will be synced in full.
Selective Sync is ON
Enabling Selective Sync allows you to sync only those files you choose explicitly. This helps to save precious space and traffic on mobile devices. Instead of transferring all, Sync will create placeholders - normally, much smaller in size, carrying basic information such as name, size and type, etc. To fetch the files in full, simply tap on them.
Files IMG_0183.jpg and IMG_0184.jpg are now fully synchronized and marked with a checkmark, whereas IMG_0185.jpg is displayed but not fully transferred yet (because it hasn't been tapped on).
Enabling Selective Sync
Android/iOS:
To enable/disable Selective Sync open the share preferences ("i" icon in main Folders view).