Marking a file as “favourite” on your Mobile Device downloads the file onto your device.
When you open a file in Dropbox, you can view the file then & there, but if you come back to it later you need to download it once again. By opening the Dropbox App on your mobile device, you can select the star icon “favourite” & the file will now be downloaded locally to your mobile device so you can view it anytime.
Marking a file as “favourite” gives you access even if you’re out of mobile coverage or on an aircraft with your device in “flight mode”.