Suppose you watch a movie or read a book on your computer.
Suddenly the display goes blank, and you have to move the mouse
or press a key to continue.
You can avoid it, if you go to the power management menu.
If your window manager is xfce4, choose "xfce4 power management" or
whatever it is called from the menu. Then click the "Display" tab,
and set the value of "Blank after" to "Never".
The best way to check if this tool is install is to open a shell
window and run the command "xfce4-power-management-settings".
If it doesn't work, install or reinstall it.