This messages will show on screen when your iCloud no more space for upload any file from iPhone or iPad. Normally You will have free use 5 gigabyte on iClound for back up file such as photo, video, document and iOS restore back up. If you want to manage for iCloud back up on iOS so just go to Settings > iCloud. You can check total available iCloud storage by go to Settings > iCloud > Storage > Manage Storage or go to iCloud Website then log in by your Apple ID. If you remove data from iCloud Website so its will auto delete from your device too.
iCloud storage will have more space if you delete or transfer some data to PC. Photos and videos are probably used most space. Its time to remove or transfer your photo and video to the other storage. If don't want to delete anything. You can buy more iCloud storage by go to Settings > iCloud > Storage > Buy More Storage.