What does screen locked mean?

Ordinarily, an incremental iCloud backup does not take long (the very first backup may take awhile, but subsequent ones will be quick).

A good practice, since iCloud only backs up once every 24 hours, is to simply plug your phone in overnight so it backs up while you sleep over your home wifi network. Then in the morning it is also fully charged and ready for you. The iCloud backup occurs in the background as long as your device is connected to an external power supply, connected to a wifi network, and the screen is in sleep/lock mode.

You can then check in settings by tapping on your user name at the top of settings, then "iCloud" then "manage storage", then "Backup" and tap on the device you wish to check to see when the last backup was performed.

if screen is locked for backup from iCloud, how can you check when it is finished backing up?

It is when the front screen is OFF (black screen) but the iPhone is ON. When using iPhone, if you press the top button (lock/power button) one time it will Lock Screen (turn screen off).