Show a lock screen.
To continue, the specified code must be entered. Allow Cancel determines whether a Cancel button will be shown or not. If Cancel is pressed the user is taken back to the home screen.
If Remember Till Off is selected, a correctly entered password will be remembered and reused until the device enters sleep mode.
A common use is to lock particular applications.
Consider locking this profile. That will prevent circumvention of this action's lock by changes within Tasker.
Also consider attaching a Lock action to any application on your system which would allow removal of Tasker e.g. system Settings.
In any case, this action should not considered as a replacement for the system Keyguard lock, as it is less secure. A reboot will remove the lock screen, for instance.