my phone says my icloud is full of backups? What are backups?

Go to Settings/[your name]/iCloud. Is iCloud Backup on? If it is, your phone is backed up every night to iCloud if it is plugged in, connected to Wi-Fi and the screen is locked. You get 5 GB of iCloud storage free; if you need more than that you can expand it to 50 GB for 99¢ a month. To take advantage of this on the same page as iCloud Backups tap Manage Storage, Change Storage Plan.