iPhone Not Backed Up
This iPhone hasn't been backed up in X weeks. Backups happen when this iPhone is connected to power, locked and on Wi-Fi.
If you run into this problem, the cause is as it says: you need to keep your phone powered, locked and on wi-fi on a regular basis. Your iPhone will try to create a backup on a daily basis when these criteria are met, and if they're not it'll alert you with this error.
It's good that Apple alert you to this, as keeping an up-to-date backup of your iOS device is a good idea: if you lose or damage the device, or accidentally delete a file, you may not otherwise be able to get it back.
If regularly leaving your phone locked, on wi-fi and powered isn't easy for you, consider switching from iCloud backups to iTunes backups.
Backing up with iTunes is free, and has a number of benefits. We publish a short guide detailing the pros and cons, as well as how to do it.