A "lock screen" is the first thing you see on your phone or computer screen before you can use it. It keeps your device secure by requiring a password, pin, or your fingerprint to unlock and access your apps and information. Full definition
Users can now adjust the background of notifications displayed on lock screen from its default opaque look to a completely invisible background. (android.gadgethacks.com)
If you have security set on your device you also get the option to hide sensitive information from lock screen notifications. (pocket-lint.com)
Even with lock screen notifications now coming as standard on Android, it's still a really useful feature. (androidcentral.com)