How do I ensure that my device's screen is locked?

If you want to back your phone up to iCloud, the recommended best practice is to put your phone on charge before you go to bed. Unless you have the screen lock set to "Never", your phone will eventually lock and the backup will start. Mine usually backs up around four or five in the morning.

Your device is locked when you need to enter your password or use Touch ID / Face ID to use the device. It is also locked when the screen is off.