Storage.

iCloud is the name of Apple's syncing service. It is dangerous to store anything of importance there as an accidental deletion from one device will delete from all devices. Check this link...

What's the difference between device storage and iCloud storage? - Apple Support

There is no other way to create space on the phone than deleting photos, messages, apps, etc.. Be sure to save important photos and data to your computer or an external drive. This video may help...

You cannot add storage. You can delete apps, data, files, photos, and videos to free space.