System Settings/Screen Locking
This module configures when to lock your screen and define the appearance of the screen locker.
The lock screen is a security feature. After some time of inactivity, plasma doesn't let anybody access your computer without your password. But this feature isn't bulletproof, for example, if your computer is stolen, the screen locker can't protect your precious file. If you want more security, you should consider an encrypted hard drive.