Screen Locking / Blanking
Your screen will automatically blank (go black) and lock after 10 minutes of no activity. That is, if you’ve made no changes to the settings.
Or, lock the screen when you choose by going to the Menu and clicking the lock icon near the top right.
Or, use a keyboard shortcut to lock the screen; the [Super] (Windows) key and the letter [L].
Prevent the screen from locking / blanking
Go to Menu > Settings > Power Manager
In the Display tab, make the following settings:
Display power management slide ON (right)
Blank after slide to 60 minutes (right)
Put to sleep after slide to 60 Minutes (right)
Switch off after slide to 60 minutes (right)
Display power management slide OFF (left)
[Close]
Go to Menu > Settings > Screensaver
In the Screensaver tab, set the following:
Enable screensaver move the slider to the ON position (right)
Activate screensaver when computer is idle move the slider to ON (right)
Regard the computer as idle after: 120 minutes (or whatever you prefer)
Inhibit screensaver for full screen applications move the slider to ON (right)
Activate screensaver when computer is idle move the slider to OFF (left)
Go to the Lock Screen tab
Enable Lock screen slide to OFF (left)
[Close]
More Info see Xfce Screensaver on Xfce Documentation and MappingWindowsKey on the Ubuntu Community Help Wiki