Backing up your device makes a complete copy of all your data, files, photos, and videos that you can use to restore a new iPhone or iPad in the case of loss or theft.
There are two ways to back up your device—synching to iCloud or saving a copy locally using iTunes. Backing up in iCloud uses Wi-Fi and your available online storage. Backing up to iTunes copies your device to your computer. Here are the differences.
Backing up to iCloud means that you are sending your device’s data to Apple’s computers for storage. iCloud Backup automatically backs up your device every day. Backup runs when you are connected to Wi-Fi, plugged into a power source, and your iPhone is locked.
Your Apple account comes with 5 GB of free space, but you can purchase more if you wish.
If you need more space, you can buy additional space or create more space in your iCloud drive by deleting existing files, photos, etc.